ADU Guide

What does it cost to build an ADU? (Part 1)

We’ve found it’s surprisingly hard to figure out what an ADU will cost before you’ve already committed to a project, paid for architectural designs and pulled your permits.

Kevin ChenSeptember 03, 2020

We’ve found it’s surprisingly hard to figure out what an ADU will cost before you’ve already committed to a project, paid for architectural designs and pulled your permits.

Why can’t you just give me a number?! We hear you, and that’s why we offer upfront cost estimates at Cottage. We’ve created this guide to transparently share all the components of costs that go into building an ADU. Costs can be divided into hard costs (labor and materials related to construction) and soft costs (all of the other expenses related to the construction project). We're starting this guide on ADU costs with soft costs first.

“Soft Costs”

Planning & eligibility

First, you need to figure out your eligibility to build an ADU on your property, which means matching up your unique property characteristics with (changing) local regulations. Some ‘ADU experts’ and prefab ADU companies charge $500 or more for a site visit. Reach out to Cottage and we’ll help consult on your ADU project for free.


Now that you know what and where on the property you can build, you’ll need to define exactly what you want your ADU to look and feel like with an architectural designer. The industry standard is 10-15% of project value/budget; in our experience, architectural designers in the Bay Area generally charge between $7,500-$20,000 in fees for designing an ADU.

Your architectural designer in turn will need to work with a few specialist consultants to build the full stamped architectural drawings that can be submitted to the city.

  • Boundary/Site Surveyor: $2,000-$3,000
  • Structural Engineering: Starting at $2,000 for a simple, single-story ADU
  • Geotechnical/soils: $5,000+
  • Title 24 calculations: $500

Make sure you know what’s included in your designs and what’s not - some architectural designers don’t provide internal elevations with their standard fees or cap the number of hours that they’ll work on your project before they start billing hourly. At Cottage, we'll work on your designs until you're satisfied. We also get great, bulk discounts with our trusted consultants that we'll pass on to you.


You’re done with your designs and ready to submit to the City. While some municipalities do split reviews between the Planning Department and Building Department, almost all charge a fee upfront for the planning and building departments to review them. Some example planning review fees in the Bay Area:

  • Palo Alto:
  • South San Francisco:
  • San Bruno:
  • Redwood City:
  • Los Altos:
  • Berkeley:

Across the Bay Area, you’ll also have to file a deed restriction at the County. As of 2020, the filing fees are:

  • San Mateo County
  • Santa Clara County
  • Contra Costa County

When you’ve answered all comments from the Planning and Building Departments and are ready to pull your permits, you’ll pay your Building Permit fees. The calculation of these fees is based on project value, design complexity, and can range between $2,000 and $5,000.

If you’ve built greater than 750 sq. ft., some municipalities will also charge Impact Fees. These fees are normally for new single-family homes - you can think of these fees as a way a city can offset the costs of the new "impact" to public services like the school district, utilities like water and sewage, and parks. For ADUs, above 750 sq. ft. in size, the fees are proportional to the size of the ADU against the size of the main house. In some cities, these impact fees can quickly become quite significant and can range from $6,000 to $25,000 for a 1,200 sq. ft. ADU.

Continue reading Part 2 about the ‘Hard Costs’ of building an ADU. Ready to go? Get started with a preliminary estimate for your project by reaching out to Cottage.

Get your free consultation and estimate today!