How Much Will My Accessory Dwelling Unit (ADU) Cost?

Sustainable NW Modern ADU in Portland, Oregon

One of the first questions we get from every client is: How much does an Accessory Dwelling Unit (ADU) cost? At the same time, we get a lot of clients that come to us with a budget and ask if we can design an ADU for that amount. The challenge for any designer is that it is impossible to answer those questions without knowing more about the unique aspects of the client's needs, project size, final design, and site conditions. However, we thought we could at least share a breakdown of the costs for one of our ADU projects to give a realistic guideline for you to base your budget on.

Below is a rough construction cost breakdown for a 790 square foot sustainable ADU that was built in the summer of 2015 in Portland, OR (we’ve inflated pricing below to reflect an average 6% construction cost increase per year since that time). This design has a few unique elements that are reflected in the cost breakdown. The design includes a high performance envelope - typical wall construction with standard batt insulation walls & roof per code, plus an additional 2” of continuous exterior insulation on all the walls, and an additional 3” of continuous exterior insulation added to the roof, plus a fully insulated slab. We also designed it to have radiant floor heating with an on-demand hot water system. There was also a couple of unique custom windows that are significantly more than standard windows would be - however they were important to the design and function of the spaces.  

Accessory Dwelling Unit (ADU) Cost Breakdown

Permits

$12,000

This includes the current Portland SDC Waiver. Typically permitting fees for an ADU fall around $6-$10k, but the additional amount here reflects the fact we had to upgrade the water supply branch & meter size at the 2015 Portland Water Bureau pricing.

Excavation

$15,000

Around a 650sf slab area including the exterior decks. Also included excavation for a couple landscaping retaining walls on the property.

Concrete Work: Footings & Retaining Walls

$28,000

This includes work needed for the ADU plus a couple of retaining walls that were designed as part of the surrounding landscaping.

Framing and Sheathing

$25,000

Typical 2x6 stud wall construction, 2x12 roof joists, supporting structural posts and beams.

Siding

$11,000

Horizontal fiber cement siding with vertical tongue & groove cedar highlights in some areas, as well as a cedar soffit.

Roofing

$13,000

Standing seam metal roof

Windows and Doors

$14,000

2 custom windows, 3 skylights, 8’ front door with side lite, 8’ two panel sliding door, and a few standard vinyl windows.

Concrete Slab

$7,000

The interior exposed concrete slab and finishing for the main level’s floor.

Plumbing

$17,700

Includes radiant floor installation and an on demand hot water system. Plumbing fixtures, sump pump, fire protection sprinklers & installation.

Electrical & Fixtures

$17,000

Electrical panel, electrical wiring and outlets. Lighting fixtures, installation, etc.

Insulation

$12,000

Batt insulation in walls and ceiling stud cavities per code, plus an additional 2” of continuous exterior insulation on all walls, 3” of continuous exterior insulation added to the roof, and a fully insulated slab on grade.

Sheetrock (Drywall)

$10,000

Drywall, mudding, taping, sanding

Interior Doors

$2,500

2 standard doors, 1 pocket door and 3 closet bi-folding doors

Paint - Exterior

$5,500

Paint - Interior

$8,000

Trim Work

$5,500

Painted MDF, plus clear coated fir window sills

Cabinets

$6,500

IKEA kitchen and some custom casework

Countertops

$3,500

Quartz countertops

Tile

$7,000

Kitchen backsplash and shower - materials and labor

Staircase

$3,500

Off the shelf spiral staircase kit

Flooring @ Loft

$3,500

Pre-finished solid White Oak

Cable Railing

$3,500

Off the shelf system

Appliances

$7,000

Stovetop, oven, washer, dryer, dishwasher, refrigerator

TOTAL

*$237,700 (last updated March, 2020)

Includes all materials, labor and contractor fees.

This is a relatively typical cost breakdown for the ADUs we work on. This isn't an extravagant project, and although there are a few places where the client invested in a bit of a premium (custom windows, skylights, radiant floor heating, spiral staircase, extra insulation), the rest of the project is pretty straightforward with standard materials. We think this is a good resource to create a relevant budget for most ADU projects.

One thing of note, is that each site condition is different. Excavation and concrete work costs can vary greatly depending on the complexity of the site (is it sloped? Lots of trees?, etc.), ease of access, and amount of fill to be added or removed. Cost can also vary depending on the exterior siding materials and interior finish materials. There were some unique aspects for the concrete work that drove up the cost and we did have some premium siding materials in certain areas of the exterior. Even then we don't really see the cost of a custom designed ADU dropping below $175,000 when all is said and done, and even to achieve that number it would have to be a small and very simple design.

It is also important to consider that a cost per square foot calculation is not a great way to determine the cost of an ADU. With smaller projects, the money that goes towards the expensive parts of a house - kitchen, bathroom, mechanical systems, excavations and concrete - don't get offset by the cheaper square footage of bedrooms, dining rooms and other spaces. For example, a 600sf ADU might cost almost as much as an 800sf ADU considering they will both have a full kitchen, a full bathroom, and all the other substantial costs: Project management, mobilization, excavation, mechanical, electrical, and plumbing systems.

If the project grows in scope beyond just a simple ADU of course the cost will also increase. A recent 2-bedroom ADU over a 2-car garage ended up at about $260,000 in construction costs. Another recent project came in right around $300,000 for an ADU lifted on stilts above a ground level storage room and covered outdoor area. As you can see, costs can vary greatly depending on the project and the client’s needs and goals.

If you have any questions don't hesitate to contact us. We are always happy to talk about the unique aspects of your projects and discuss your ideas and goals for a sustainable Accessory Dwelling Unit. If you would like to see some of our other ADU designs, please click here.