ADU Cost Estimate

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 an ADU can be built for that amount. The problem for any designer is that it is impossible to answer that question without knowing more about the unique aspects of the design that addresses the client's needs, wants, and site conditions. However, we thought we should at least share a breakdown of the costs for a recent ADU project to give a realistic guideline for you to base your budget on.

Below is a rough construction cost breakdown for a 795 square foot sustainable ADU that was built in the summer of 2015 in Portland, OR. 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, plus an additional 2” of rigid insulation on all the walls, 3” of rigid added to the roof, and a fully insulated slab. We also designed in radiant floor heating with an on-demand hot water system. There was also a couple of unique custom windows that are significantly more than a standard window would be - however they were important to the design and function of the spaces.  Finally, this breakdown is for the construction costs and doesn't include architectural or structural design work. The design fees vary depending on the complexity, size and budget of your project. 

Accessory Dwelling Unit (ADU) Cost Breakdown:



This includes the current Portland SDC Waiver that is expiring in July and all of the individual trade permits. Usually the permitting for the architectural scope is around $5000, with the remainder amount for the individual trade permits (plumbing, electrical, etc.)



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

Footings & Retaining Walls


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

Framing and Sheathing


typical 2x6 stud wall construction and 2x12 roof joists and supporting structural posts and beams



Hardi siding with vertical tongue and groove cedar highlights in some areas and cedar soffit



Standing seam metal roof

Windows and Doors


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

Concrete Slab


exposed concrete slab for floor of main level



includes radiant floor installation and on demand hot water system



electrical panel, electrical wiring and outlets



batt insulation in walls and ceiling stud cavities, 2" rigid on exterior walls and 3" rigid on roof

Sheetrock (Drywall)


drywall, mudding, taping

Interior Doors


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

Paint - Exterior


Paint - Interior


Trim Work


painted MDF, plus clear coated fir window sills



Ikea kitchen and some custom work



Quartz counters



Kitchen backsplash and shower - materials and labor



off the shelf spiral staircase

Flooring - loft


solid white oak, prefinished

Cable Railing


off the shelf system



stove, oven, washer, dryer, refrigerator

Electrical Fixtures


lighting fixtures, installation, etc.

Additional Plumbing


plumbing fixtures, installation, etc.



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, etc.), the rest of the project is pretty straightforward. I think this is a good resource to base your project's budget on.

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, ease of access, and amount of fill to be added or removed. Cost can also rise dramatically 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 $150,000 when all is said and done.

It is also important to consider that a cost per square foot calculation is not a great way to determine the cost of a project like this. 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.

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 click here.