Folsom's ADU Rules in 2026: Sizes, Setbacks, and the 60-Day Clock

Accessory dwelling units have gone from a zoning curiosity to one of the most common projects we discuss with Folsom homeowners. The reason is straightforward: California has spent several years systematically removing the local barriers that used to make ADUs impractical, and Folsom has now brought its own code fully into line.

If you investigated an ADU two or three years ago and concluded it was not workable, it is worth looking again.

What changed

The City of Folsom adopted a completely rewritten ADU ordinance — Ordinance 1361, codified at Chapter 17.105 of the Folsom Municipal Code — which took effect May 14, 2026. It aligns the city's requirements with current state law and, importantly, folds design review into the building permit rather than running it as a separate discretionary process.

That last point is the practical headline. Design review used to be where ADU projects stalled for months. It is now part of the building permit path.

Size limits

Under the current Folsom rules:

• 850 square feet maximum for a unit with one bedroom or fewer
• 1,000 square feet maximum for a unit with two or more bedrooms
• 150 square feet minimum
• An 800 square foot ADU with four-foot setbacks can never be precluded — this is a state-law floor the city cannot zone away

State law in 2026 permits detached ADUs up to 1,200 square feet on many residential lots; local ordinances set the operative limit within that. Confirm your specific parcel with the city.

Setbacks and parking

Setbacks are four feet from side and rear property lines for new construction. Conversions of existing structures — turning a detached garage or a portion of the house into an ADU — have no setback requirement, which is what makes garage conversions so attractive on tighter lots.

Parking is generally one space per unit, with meaningful exemptions under state law, including proximity to transit and conversions of existing space.

The 60-day clock

Folsom's ordinance codifies the state's 60-day deemed-approval rule. Once you submit a complete ADU application, the city has 60 days to approve or deny it. If it does neither, the application is deemed approved by operation of law.

The operative word is complete. An incomplete submittal does not start the clock. This is precisely why a well-prepared first submittal matters more on ADUs than on almost any other project type.

Fee relief worth knowing about

Recent state legislation has continued to reduce ADU fee burdens — including exempting units under 500 square feet from school impact fees. Impact fees historically added tens of thousands to ADU budgets, so these exemptions materially change the math on smaller units.

What an ADU actually costs to build

Ordinance compliance is the easy part. Construction is where the budget lives. The main cost drivers on a Folsom-area ADU:

• Utility connections. Running water, sewer, gas, and electrical to a detached structure at the back of a lot is trenching work, and distance costs money. Whether your existing electrical panel can carry the additional load is one of the first things to determine.
• Foundation. Much of the Folsom area sits on expansive clay soil that moves seasonally. That has real foundation implications — see our soil guide.
• Conversion versus new build. Converting an existing garage is almost always cheaper than building new, because the foundation, walls, and roof already exist.
• Site access. If a concrete truck cannot reach the build site, everything gets more expensive.

Is an ADU right for your lot?

The honest answer for some properties is no. Lot geometry, existing utility locations, mature trees, grade, and access can combine to make an ADU technically permissible but economically poor. We would rather tell you that at the walkthrough than at the framing stage.

For the lots where it works, an ADU is one of the highest-value things you can build on a Folsom property — whether for multigenerational living, rental income, or long-term flexibility.

Read more on our ADU construction page, and confirm current requirements with the City of Folsom's ADU resources.

Related reading

• Expansive Clay Soil and Your Foundation: Building in the Sierra Foothills
• Folsom Building Permits for Remodels: Process, Timelines, and Fees
• How to Vet a Remodeling Contractor in the Folsom Area

Planning a project in the Folsom area?

Mohr Construction has been remodeling homes across Folsom, El Dorado Hills, Granite Bay, Roseville, Rocklin, Loomis, Fair Oaks, Orangevale, and Rancho Cordova since 2004 — and we have grown entirely through referrals. We communicate daily, we take on the complicated projects other builders pass on, and we do not cut corners.

Start a conversation about your project or try our remodel cost calculator to get a ballpark before you talk to anyone.