Do you need a site plan for an ADU permit? In many cities and counties, the answer is yes. An accessory dwelling unit adds another residential unit to a property, so permit reviewers need to understand exactly where it will be located and how it relates to the primary house, property lines, utilities, access, drainage and other site conditions.

An ADU permit site plan, sometimes called an ADU plot plan, is a scaled drawing of the property viewed from above. It can show the proposed ADU, existing house, lot dimensions, setbacks, driveways, parking, walkways, easements, sewer and water connections, stormwater drainage and other important details.

The site plan is normally submitted with architectural and structural plans. Those additional drawings explain the ADU’s rooms, foundations, framing, roof, windows, doors, plumbing, electrical systems and construction details.

Requirements vary by jurisdiction. Portland explicitly says all ADU construction projects need a site plan. Seattle requires construction permits for attached and detached ADUs, while Denver requires zoning, building and sewer-use-and-drainage permits for all ADUs. [1][2][3][4]

What Is an ADU?

An ADU is a smaller, independent dwelling located on the same property as a primary residence.

Depending on the jurisdiction, ADU may mean accessory dwelling unit or additional dwelling unit. Other common names include:

• Backyard cottage
• Granny flat
• In-law suite
• Mother-in-law apartment
• Garage apartment
• Carriage house
• Casita
• Secondary dwelling unit

An ADU normally contains its own space for sleeping, living, cooking and sanitation.

Denver describes ADUs as self-contained living spaces with their own kitchen, bathroom and sleeping area. Austin treats a structure or part of a structure as a dwelling unit when it contains habitable space, a full bathroom and a sink or dishwasher outside the bathroom. [4][5]

What Types of ADUs May Require a Site Plan?

Site-plan requirements can apply to several types of ADU projects.

Detached ADU

A detached ADU is a separate building located elsewhere on the property, usually in the side or rear yard.

Examples include:

• Backyard cottage
• Detached granny flat
• Prefabricated ADU
• Tiny house installed on a permanent foundation
• New living unit beside or behind the primary house

A detached ADU site plan is especially important because the reviewer must check its location, setbacks, height, building separation, utilities and access.

Attached ADU

An attached ADU is physically connected to the primary house.

It may be:

• A new side or rear addition
• A new wing connected to the house
• A unit above an attached garage
• A converted portion of an existing addition

The site plan should show the complete house footprint and clearly identify the new attached portion.

Internal ADU

An internal ADU is created within the existing house.

Common examples include:

• Basement apartment
• Attic conversion
• Converted bedroom wing
• Separate unit within an existing floor

A site plan may still be required even when the exterior footprint does not change. Portland, for example, requires a site plan for ADU construction projects and lists a site plan among the materials needed when converting existing space into an ADU. [1]

Garage Conversion or Garage Apartment

An ADU may be created by:

• Converting an existing detached garage
• Converting part of an attached garage
• Adding an apartment above a garage
• Replacing a garage with a new garage-and-ADU building

The site plan should show the garage, driveway, parking, building setbacks, utilities and relationship to the main house.

Our guide on site plans for garage permits explains related garage-location and access requirements.

Does Every ADU Need a Permit?

An ADU creates legal residential living space and will normally require building approval.

Additional approvals may include:

• Zoning or land-use approval
• Site-development approval
• Electrical permit
• Plumbing permit
• Mechanical permit
• Sewer-use or utility approval
• Stormwater or drainage approval
• Address assignment
• Health-department approval for septic properties
• Historic-district approval

Seattle requires an addition or alteration construction permit when an ADU is added within an existing house or built as a detached unit. Legalizing an existing unapproved unit requires a permit to establish the use. [2]

Denver states that all ADUs need zoning, building and sewer-use-and-drainage permits. [4]

Austin requires an ADU to comply with applicable dwelling-unit rules involving zoning, utilities, occupancy and addressing. Austin also requires each new dwelling unit to have a unique address or building number before the residential permit application is submitted. [5]

Why Do Permit Reviewers Need an ADU Site Plan?

The site plan allows reviewers to see how the proposed dwelling fits on the property.

It helps them evaluate:

• Whether an ADU is allowed on the property
• Where the ADU will be located
• Required front, side and rear setbacks
• Distance between the ADU and primary house
• Maximum building height
• Lot coverage and impervious area
• Parking and vehicle access
• Pedestrian access to the ADU
• Fire separation between buildings
• Water, sewer and electrical connections
• Stormwater drainage
• Easements and underground utilities
• Tree protection and environmental constraints

Denver’s current application instructions require a scaled site plan showing lot lines, adjacent streets and alleys, existing structures and driveways, the proposed ADU, its height, and distances to buildings and lot lines. [4]

What Should a Site Plan for an ADU Permit Show?

The exact checklist depends on the permitting authority and property. However, a useful ADU site plan commonly includes the following information.

Property Boundaries and Lot Dimensions

Show the complete legal outline of the property.

Include:

• Front property line
• Side property lines
• Rear property line
• Length of each boundary
• Lot width and depth
• Irregular or angled boundaries
• Property address

The property lines provide the reference points for measuring the ADU setbacks.

Do not automatically assume that a fence, hedge or landscaped edge marks the legal boundary. A boundary survey may be needed when the ADU will be constructed close to a property line.

Street, Alley and Right-of-Way

Show all streets and alleys adjoining the property.

Include where relevant:

• Street name
• Public alley
• Private alley
• Sidewalk
• Curb or road edge
• Public right-of-way
• Second street on a corner lot

Alleys are especially important when the ADU, garage or parking area will be accessed from the rear of the property.

Existing Primary House

Show the complete footprint of the existing house and label it clearly.

The plan may show:

• Exterior walls
• Roof outline
• Porches and decks
• Attached garage or carport
• Existing additions
• Doors facing the proposed ADU
• Windows near the proposed building

The house should be labelled:

Existing Primary Dwelling

Other Existing Structures

Show other structures and improvements that may affect the ADU.

These can include:

• Detached garage
• Carport
• Shed
• Deck or patio
• Swimming pool
• Retaining walls
• Water tanks
• Fences and gates
• Outdoor mechanical equipment
• Existing accessory buildings

Leaving an existing structure off the plan may produce inaccurate setback, building-separation or lot-coverage calculations.

Proposed ADU Location

Clearly identify the new unit with a label such as:

Proposed Detached Accessory Dwelling Unit

Use a bold outline, hatching or different line type so the proposed ADU can be distinguished from existing buildings.

The drawing should clearly separate:

• Existing structures
• Structures proposed for removal
• New proposed construction

ADU Dimensions and Floor Area

Show the overall exterior dimensions of the proposed unit.

Include:

• Overall length
• Overall width
• Total floor area
• Number of stories
• Covered porch or entry
• Exterior stairs
• Roof projections
• Attached garage or storage area

A useful note might read:

Proposed Detached ADU: 25 ft × 26 ft — 650 sq. ft. — One Story

The dimensions should match the floor plan, foundation plan, roof plan and building elevations.

Front, Side and Rear Setbacks

Show the shortest measured distance from the proposed ADU to each nearby property line.

Include:

• Front setback
• Left-side setback
• Right-side setback
• Rear setback
• Corner-side setback where applicable
• Distance from roof overhangs where required

Setback rules vary according to the city, zoning district, ADU type, building height and whether the structure is new or converted.

Portland notes that side and rear setbacks can start at five feet but may be larger depending on the property’s zoning. This is a Portland-specific example and should not be treated as a nationwide ADU setback. [1]

Distance Between the ADU and Primary House

Show the measured distance between the ADU and the main house.

This distance may affect:

• Fire separation
• Exterior-wall construction
• Roof and eave projections
• Window and door locations
• Emergency access
• Maintenance access
• Drainage between buildings

Austin notes that its zoning regulations no longer impose a minimum distance between units, but applicable technical-code separation requirements still apply. Other jurisdictions may use different rules. [5]

ADU Height

Show the proposed building height.

Relevant measurements can include:

• Wall or plate height
• Maximum roof height
• Average roof height
• Roof pitch
• Finished floor elevation
• Height above natural grade

Denver specifically asks applicants to include the proposed ADU height on the site plan and provide dimensional building elevations. [4]

Attached or Detached Design

Clearly identify whether the unit is:

• Inside the primary house
• Attached to the primary house
• Detached from the primary house
• Above a garage
• Inside a converted garage

This affects the permit pathway, fire separation, foundations, building classification and utility design.

ADU Entrance and Walkway

Show how residents and visitors will reach the unit.

The plan may include:

• ADU entrance
• Walkway from the street
• Side-yard path
• Route from the driveway
• Steps and landings
• Ramp where proposed
• Gates
• Exterior lighting locations where relevant

The walkway should provide a practical route without requiring occupants to walk through landscaping, squeeze between buildings or cross a parking space that will normally be occupied.

Address and Building Identification

Some jurisdictions require a separate address or building number for the ADU.

Austin currently requires each new dwelling unit to have a unique address or building number before the residential building-permit application is submitted. [5]

The site plan may show:

• Primary-house address
• Proposed ADU address
• Location of address numbers
• Pedestrian route used by emergency responders

Driveway and Vehicle Access

Show the existing driveway and any proposed changes.

Include where applicable:

• Existing driveway
• Proposed driveway extension
• New driveway
• Driveway width
• Garage or carport access
• Alley connection
• Curb cut
• Vehicle turning area

Denver requires new driveways to be shown with their location, width, construction material and dimensions to property lines and neighbouring driveways. [4]

See our guide on site plans for driveway permits when the ADU project changes the property’s vehicle access.

Parking Spaces

Parking requirements vary greatly by jurisdiction and location.

Where parking must be documented, show:

• Existing garage spaces
• Existing driveway parking
• Proposed ADU parking
• Parking-space dimensions
• Vehicle-access route
• Required turning or manoeuvring area

Do not add parking to the site plan merely because another jurisdiction requires it. Check the current rules applying to the property.

Lot Coverage and Building Coverage

An ADU adds another building footprint or expands the existing dwelling.

Where required, the site plan may show:

• Total lot area
• Primary-house footprint
• Garage and accessory-building areas
• Covered porches and patios
• Proposed ADU footprint
• Total building coverage
• Maximum permitted coverage

Coverage definitions vary. Roofed patios, eaves, decks and accessory structures may be treated differently by different zoning codes.

Floor-Area or Density Calculations

Some jurisdictions regulate the ADU through:

• Maximum ADU floor area
• Floor-area ratio
• Maximum number of dwelling units
• Relationship between the ADU and primary dwelling
• Minimum lot-size requirements

Austin’s current ADU guidance says the allowable unit size depends on the zoning district and geographic location. It also identifies minimum lot-area and zoning criteria for its ADU process. [5]

Do not assume that a common size such as 650, 800 or 1,000 square feet is permitted everywhere.

Impervious Area

A detached ADU can increase the property’s impervious area through:

• Roof area
• New driveway
• Parking pad
• Concrete walkway
• Patio
• Other hard surfaces

Where required, show the existing and proposed impervious areas and the remaining landscaped or permeable space.

Austin explains that site review can consider zoning, drainage, floodplain, water quality, transportation, erosion control, environmental issues and tree protection. [9]

Easements

Show every known easement affecting the property.

Common examples include:

• Utility easement
• Drainage easement
• Sewer easement
• Stormwater easement
• Access easement
• Shared-driveway easement

Label the width and purpose of each easement.

Do not place the ADU, foundations, eaves, stairs or retaining walls inside an easement without confirming that construction is allowed.

Water Connection

Show the proposed water-service arrangement where requested.

The ADU may use:

• A shared water meter
• A separate water meter
• A branch from the existing house service
• A new service from the public water main

Portland explains that a shared meter can serve the house and ADU, but the combined plumbing fixtures must be considered when determining whether the existing meter is large enough. A separate meter can also be used. [6]

Sanitary Sewer Connection

Show known sewer information where required.

This may include:

• Existing sewer lateral
• Proposed ADU sewer line
• Connection point
• Cleanouts
• Pipe size and direction
• Public sewer main
• Sewer easement

Denver asks applicants to show the size, type and location of sanitary-sewer lines on the site plan for sewer-use-and-drainage review. [4]

Stormwater Connection and Roof Drainage

A detached ADU adds roof area and can change how stormwater moves across the lot.

The plan may show:

• Roof-fall direction
• Gutters
• Downspouts
• Stormwater pipes
• Drywell or infiltration system where permitted
• Surface-flow arrows
• Lawful discharge point

Portland has separate guidance for ADU water, sanitary-sewer and storm-sewer connections. Denver requires storm-sewer information on the ADU site plan for its drainage review. [4][6]

Electrical and Gas Services

Show utility equipment and service routes when requested.

These may include:

• Existing electrical meter
• New electrical meter or subpanel
• Underground electrical route
• Overhead power lines
• Gas meter
• Gas-service route
• Utility poles

Seattle notes that an ADU project may also require approval for electrical service changes or new service. [2]

Fire Separation

The site plan should show the distance between the ADU and other buildings so the reviewer can evaluate applicable fire-separation rules.

Fire-related design can affect:

• Exterior-wall ratings
• Roof and eave construction
• Window and door locations
• Fire-resistant projections
• Emergency access

Austin states that applicable fire-separation requirements must be followed for its ADU projects. Denver asks for fire-rated wall and projection details as part of the building-plan package. [4][5]

Existing and Proposed Site Levels

Show levels where the site slopes or the ADU requires grading.

The plan may include:

• Existing contours
• Natural ground levels
• Proposed finished floor elevation
• Finished ground levels
• Direction of site fall
• High and low points
• Walkway and driveway levels

Building elevations and sections usually provide more detailed vertical information.

Cut, Fill and Grading

Show earthwork required to create the ADU building pad.

This may include:

• Excavation area
• Depth of cut
• Imported fill
• Finished building pad
• Slopes and batters
• Drainage around the building

Substantial grading may require additional civil, drainage or geotechnical information.

Retaining Walls

Show retaining walls located near the proposed ADU.

Include:

• Existing walls
• Proposed walls
• Wall length
• Maximum wall height
• Distance to the ADU
• Distance to property lines
• Drainage behind the wall

The ADU foundation should not place an unassessed load on an existing retaining wall.

Trees and Landscaping

Show significant trees where construction may affect them.

The site plan may identify:

• Tree-trunk locations
• Trees to remain
• Trees proposed for removal
• Tree-protection areas
• Street trees
• Required replacement planting
• Limits of construction disturbance

Portland advises applicants to consider street trees when planning ADU utility connections. Charlotte’s residential-lot review may also include tree-preservation review for ADUs. [6][8]

Septic System and Private Well

Properties without public sewer or water may need to show:

• Septic tank
• Drain field
• Reserve disposal area
• Private well
• Required separation distances
• Proposed ADU connection

Adding a dwelling unit can increase wastewater demand. Health-department approval or a septic-system evaluation may be needed.

Construction Access

Show how construction equipment and materials will reach the ADU location where access is constrained.

The plan may include:

• Driveway access
• Side-yard access
• Gate width
• Temporary construction path
• Material-storage area
• Tree-protection fencing
• Limits of disturbance

North Arrow, Drawing Scale and Plan Information

The site plan should include:

• North arrow
• Recognized drawing scale
• Property address
• Drawing title
• Date prepared
• Revision number

Possible residential site-plan scales include:

• 1 inch = 10 feet
• 1 inch = 20 feet
• 1 inch = 40 feet for a larger property

Denver specifically requires the ADU site plan to be drawn to scale and include both the scale and a north arrow. [4]

Do Pre-Approved ADU Plans Eliminate the Need for a Site Plan?

No. Pre-approved building plans can simplify review of the ADU structure, but the design still has to be placed on a specific property.

The permitting authority may still need to review:

• Property boundaries
• Setbacks
• Building height
• Lot coverage
• Utilities
• Drainage
• Parking and access
• Trees and easements
• Site-specific foundation conditions

Portland says all ADU construction projects need a site plan, including projects using its pre-approved detached ADU designs. The city also warns that the pre-approved designs are not suitable for every lot. [1]

Seattle offers pre-approved detached ADU designs as a faster and more predictable permitting pathway, but applicants still go through the property-specific permit process. [2]

Can an Existing Garage Be Converted Into an ADU?

Possibly, but the garage must be evaluated for residential use.

The conversion may require upgrades involving:

• Foundation and slab
• Wall and roof framing
• Ceiling height
• Insulation and energy efficiency
• Natural light and ventilation
• Emergency escape openings
• Fire separation
• Plumbing and sanitary drainage
• Heating and electrical systems
• Weather protection

The site plan should show the existing garage, its setbacks, the driveway, parking, utilities and relationship to the primary house.

Can an ADU Be Built Above a Garage?

An above-garage ADU may be allowed where the applicable zoning and building codes permit it.

The application may need to address:

• Total building height
• Exterior stairs
• Structural capacity
• Fire separation between garage and dwelling
• Emergency escape and rescue openings
• Parking and driveway access
• Privacy and window placement
• Utility connections

The site plan shows the garage-and-ADU footprint, while elevations and sections explain the second-story living space.

Site Plan vs ADU Floor Plan

The two drawings provide different information.

ADU Site Plan

The site plan shows:

• The entire property
• Property boundaries
• Primary house and other structures
• ADU location and dimensions
• Setbacks
• Driveways and parking
• Walkways and entrances
• Utilities and drainage
• Easements and site features

ADU Floor Plan

The floor plan shows:

• Bedrooms and living areas
• Kitchen
• Bathroom
• Doors and windows
• Interior and exterior walls
• Room dimensions
• Plumbing fixtures
• Stairs where applicable

The site plan explains where the ADU goes. The floor plan explains how the ADU is arranged inside.

What Other Drawings May Be Required?

A site plan is normally only one part of the ADU permit package.

The permitting authority may also request:

• Existing-condition plan
• Demolition plan
• Proposed floor plan
• Foundation or slab plan
• Roof plan
• Front, side and rear elevations
• Building sections
• Wall and roof-framing plans
• Structural calculations
• Fire-separation details
• Energy-code documents
• Window and door schedule
• Electrical plan
• Plumbing plan
• Mechanical plan
• Utility-connection plan
• Drainage or grading plan

Denver requires building elevations, structural plans, foundation details, roof information, wall construction and fire-rated projection details in addition to the site plan. [4]

Do You Need a Survey for an ADU Permit?

Not every ADU application requires a new boundary survey. An existing survey, mortgage-location plan or recorded plat may provide a usable base.

A new or updated survey may be required when:

• The ADU is close to a property line
• Property corners cannot be located
• The fence may not follow the legal boundary
• The lot has an irregular shape
• An easement location is uncertain
• Certified elevations are needed
• The property is in a floodplain
• The permitting authority specifically requests one

Denver currently requires a property survey when proposed ADU construction is within three feet of a property line. It requests a boundary survey for vacant land or an improvement survey where structures already exist. That is a Denver-specific requirement. [4]

A drafted permit site plan is not a legal boundary survey and should not be represented as one.

Can You Use an Online Map as the ADU Site Plan?

An aerial image can help identify the general property layout, but it is usually not sufficient as the complete permit drawing.

Online imagery may be:

• Outdated
• Distorted
• Missing legal property lines
• Missing reliable dimensions
• Missing easements and utilities
• Missing site levels
• Unable to show the proposed ADU accurately

A permit site plan should be drawn consistently, include written dimensions and clearly distinguish existing conditions from proposed work.

Common ADU Site Plan Mistakes

ADU applications can be delayed when the site plan is incomplete or inconsistent with the other drawings.

Common mistakes include:

• Property lines are missing
• The proposed ADU is not clearly labelled
• ADU dimensions or floor area are missing
• Boundary setbacks are omitted
• Roof overhangs are ignored
• The distance to the primary house is missing
• Building height is not shown
• Attached or detached construction is unclear
• The walkway and entrance are omitted
• Driveway or parking changes are not shown
• Lot coverage is missing where required
• Easements are ignored
• Water, sewer or electrical connections are unclear
• Stormwater drainage is not addressed
• Site grading is missing
• Retaining walls are omitted
• Trees or septic systems are ignored
• The drawing is not to scale
• There is no north arrow
• The site plan conflicts with the floor plans or elevations

What Information Is Needed to Prepare the Site Plan?

Collect as much of the following information as possible:

• Property address
• ADU length and width
• Total floor area
• One-story or two-story design
• Attached, detached or conversion design
• Preferred ADU location
• Distance to nearby property lines
• Distance to the primary house
• ADU entrance and walkway location
• Driveway and parking layout
• Photos of the property
• Rough sketch with measurements
• Existing survey or plot plan
• Recorded plat if available
• Architect, builder or manufacturer drawings
• Known easements
• Water, sewer and electrical information
• Site levels or contours
• Drainage information
• Retaining-wall information
• Permit-review comments

ADU Permit Site Plan Checklist

Does a Site Plan Guarantee ADU Permit Approval?

No. A clear site plan gives reviewers the information needed to understand the proposed location, but approval also depends on the zoning code, structural design, building code, utilities and property conditions.

The project may also require:

• Architectural and structural plans
• Licensed boundary survey
• Zoning or land-use approval
• Electrical, plumbing and mechanical permits
• Sewer and water approval
• Stormwater or grading approval
• Fire-department review
• Floodplain or environmental review
• Historic-district approval
• Tree-removal approval
• Septic or health-department approval
• HOA or architectural-review approval

Need a Site Plan for an ADU Permit?

Site Plan Pros prepares clear residential site plans for homeowners, contractors, designers and permit applications.

Your ADU site plan can show:

• Property boundaries
• Existing house and structures
• Proposed ADU location
• ADU dimensions and setbacks
• Building separation
• Driveway, parking and pedestrian access
• Lot coverage and impervious area
• Water, sewer and electrical connections
• Drainage and grading
• Easements and other property details

You may also find our guides on home addition permit site plans, garage permit site plans, driveway permit site plans and site plans versus surveys helpful.

Confirm the local ADU, setback, utility, drainage and permit requirements before finalising the design or beginning construction.

Sources

1. City of Portland — Accessory Dwelling Units and Permit Requirements
2. Seattle Department of Construction and Inspections — Accessory Dwelling Units
3. Seattle Department of Construction and Inspections — Getting a Residential Dwelling Unit Permit
4. City and County of Denver — Permitting an Accessory Dwelling Unit
5. City of Austin — Additional Dwelling Units
6. City of Portland — ADU Water, Sanitary Sewer and Storm Sewer Connections
7. City of Charlotte — Accessory Dwelling Unit Guidebook
8. City of Charlotte — Individual Residential Lot Reviews
9. City of Austin — Site Plans, Exemptions and Corrections
10. City of Portland — ADU Zoning Requirements