What is the size limit for an ADU in Spokane County?

Size limits are set by your jurisdiction. Inside an urban growth area, HB 1337 requires that an ADU of at least 1,000 square feet be allowed. Confirm the current cap on the relevant jurisdiction page before planning, or with Spokane County Building and Planning for a specific unincorporated parcel.

How many ADUs can I have on one lot under HB 1337?

Within an urban growth area, HB 1337 (2023) requires covered jurisdictions to allow at least two ADUs on lots that permit single-family homes. The exact count for your lot depends on local implementation, so confirm the current number with your jurisdiction. Our HB 1337 guide explains the law.

Guide

Can I build an ADU on my lot in Spokane?

Most Spokane-area homeowners can build some form of ADU, but feasibility depends on a handful of factors. Work through this checklist, then get matched with a licensed local builder who can confirm it for your specific lot.

Check my lot Rules by jurisdiction

Short answer: most Spokane-area residential lots can support at least one ADU, and often two. Washington's 2023 HB 1337 requires the City of Spokane, Spokane Valley, and the urban growth area of Spokane County to allow up to two ADUs on lots that permit single-family homes, with no owner-occupancy requirement (RCW 36.70A.681). Whether your specific lot works comes down to your jurisdiction, lot size and setbacks, and utility access. Work through the five checks below, then get matched with a builder who can confirm it for your parcel.

1. Identify your jurisdiction

ADU rules differ across the City of Spokane, unincorporated Spokane County, and Spokane Valley. Start by confirming which one governs your lot, because everything else follows from it. If your parcel is not inside a city, the county rules apply.

2. Check zoning and allowed ADU types

Most residential zones allow ADUs, but the allowed types and counts depend on your jurisdiction and may have changed after HB 1337. Confirm what your zone allows.

Inside an urban growth area, HB 1337 requires jurisdictions to allow at least two ADUs per lot, detached or attached, on lots that permit single-family homes (RCW 36.70A.681). Your exact zone rules are on your jurisdiction page: City of Spokane, Spokane County, or Spokane Valley.
The four types differ in what they cost and what your lot has to support: a new detached ADU, an attached ADU or addition, a garage conversion, or a basement ADU or JADU.

3. Measure your lot against setbacks and size

A buildable ADU has to fit within the setbacks and size limits for your jurisdiction while leaving room for access and any required separation from the main house.

HB 1337 requires jurisdictions to allow an ADU of at least 1,000 square feet and a roof height of at least 24 feet (RCW 36.70A.681). Your jurisdiction's exact setbacks and size caps are on its rules page, cited to the current local code.

4. Confirm utility access

Water, sewer or septic, and power all need to reach the unit. On city lots this is usually a connection question. On county lots, septic capacity can be a deciding factor and is worth checking early.

5. Account for parking and HB 1337

Parking requirements have been affected by HB 1337, especially near major transit. Confirm the current parking rule for your jurisdiction rather than assuming an older standard. Our HB 1337 guide explains the baseline.

Feasibility checklist

I know which jurisdiction governs my lot.
My zone allows the ADU type I want.
My lot can fit the unit within setbacks and size limits.
Water, sewer or septic, and power can reach the unit.
I understand the current parking rule for my jurisdiction.

If you can check most of these, you are likely a good candidate. The fastest way to confirm is to get matched with a builder who serves your jurisdiction.

Find out what your lot can support

Share a few details and we will connect you with a licensed local builder who serves your jurisdiction. Free to you, no obligation.

Name

Phone

Your jurisdiction

Project type

Property address or nearest cross-streets (optional)

Helps the builder check what your specific lot can support. Skip it if you would rather not share it yet.

Lot or timeline notes (optional)

I agree to be contacted by Compass Lead Group and the licensed local builder I am matched with, by phone and email, about my ADU project. Consent is not a condition of any purchase, and message and data rates may apply. I can opt out at any time. See the Terms and TCPA disclosures and the Privacy policy.

We do not promise a specific price, timeline, financing approval, or permit outcome.

Feasibility questions

Can I build an ADU on my lot in Spokane?: Most residential lots in the Spokane area can support some form of ADU, but the answer depends on your jurisdiction, lot size and shape, setbacks, and utility access. This checklist helps you assess feasibility, and the builder you are matched with confirms it for your specific property.
What is the size limit for an ADU in Spokane County?: Size limits are set by your jurisdiction. Inside an urban growth area, HB 1337 requires that an ADU of at least 1,000 square feet be allowed. Confirm the current cap on the relevant jurisdiction page before planning, or with Spokane County Building and Planning for a specific unincorporated parcel.
How many ADUs can I have on one lot under HB 1337?: Within an urban growth area, HB 1337 (2023) requires covered jurisdictions to allow at least two ADUs on lots that permit single-family homes. The exact count for your lot depends on local implementation, so confirm the current number with your jurisdiction. Our HB 1337 guide explains the law.

References and primary sources

Regulatory and fee specifics on this page are cited to the current primary sources below. Confirm any figure against its source for your specific parcel before you rely on it.