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  • Lorianna Kastrop

ADU Costs: A Reality Check

By Lorianna Kastrop, Vice President/CFO, The Kastrop Group, Inc. Architects

You want to build an Accessory Dwelling Unit on your residential property. How much will it cost? That’s the big question and it’s difficult to answer. I decided that I would share with Kastrop Group blog followers the general costs that we paid for an ADU on our own property. I am showing the costs paid by category, knowing that your own local providers may be able to give you more competitive bids. But this article will give you an idea of what this type of construction currently costs on the mid-Peninsula in the San Francisco Bay Area. Please understand that there is some leeway in what you decide to spend, especially on the finishes. Do not be discouraged if these costs are higher than what you had in mind.

It is also important to note that we were building during the COVID-19 epidemic. Materials prices, especially lumber, have soared due to the interruption of supply chains and the ravages of west coast wildfires over the past few years. The contractors may have taken longer (more labor cost) to complete the work, because to maintain social distance requirements there were fewer workers on the site.

Another factor that increased the cost for us was having a fully handicap-accessible ADU that has lots of storage (custom cabinets). We designed it to be suitable for us to live in after retirement if we should wish. That resulted in choosing elements that include vaulted ceilings, clerestory windows, more elegant roof style and double thick walls. Therefore, we did not always choose more economical solutions that would be adequate for a rental unit.

Photo of our ADU, taken by Mike Kastrop.

On the other hand, we got preferred pricing as a professional courtesy from the engineers and the general contractor because we have worked together on many projects. Since our Principal Architect D. Michael Kastrop, AIA was working on this project for himself, his time was heavily discounted as well. Normally the Professional Fees are about 8-12% of the construction cost. For us they were less than 5%.

We spent $470 per square foot to build our ADU.  I know, that sounds like a crazy amount of money.  But here’s how it broke down:

Permits and fees:


includes planning & building permits, school impact fees, new electrical meter, new street address processing fee

Professional fees:


includes architect, surveyor, structural engineer, mechanical/electrical/plumbing engineer, Title 24



includes tree removal, landscape repair & planting after construction

General Contractor:


includes sub-contractors for custom cabinets and painting

Solar system:


includes panels & a high-capacity home battery

Purchase & Installs:


appliances, hardware, light fixtures, doors, windows, skylights, countertops, tile, accessibility hardware, flooring, purchased by owner and installed by contractor



for a 699 square foot ADU

Note: These numbers do not include any contents or furnishings.

It is always a big step to start a construction project.  Plan ahead and arrange your financing so that you don’t get caught short.  We usually tell our clients to add a 10 to 15% contingency to cover unanticipated costs.  In this case, we initially budgeted $300,000, so we did fall within that +10% margin.  The bottom line is that ADU’s cost more per square foot than ordinary residential construction.  See our website for more photos of the completed ADU at:

Thanks for reading and as always, we are “Designing for Your Reality”.

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