Building an ADU in Northern California
An accessory dwelling unit is a small house, and small houses are expensive per square foot: one kitchen, one or two baths, a full envelope and a full set of utilities, spread across 700 sq ft instead of 4,000. Here is what that actually costs around Sacramento, what California law allows, and how long it takes.
Three ways to add a second unit
Garage conversion
Lightest scope
Existing slab and structure, new envelope, mechanical, kitchen and bath. Fastest route to a rentable second unit when the garage is already sound.
Detached ADU (600 to 900 sf)
Most common scope
Ground-up guest house on its own foundation with its own utility service. The most common scope we build, and the one that adds the most value on a Placer County parcel.
Two-story garage apartment
Heaviest scope
Parking below, living above. More structure and stair, but it preserves yard on a tighter Rocklin or Roseville lot.
What the code allows
- Lot size
- California law allows at least one ADU on virtually any lot zoned for residential use. Local minimum lot size rules cannot be used to block one.
- Size cap
- Jurisdictions must allow at least 850 sf, or 1,000 sf for a two-bedroom unit. Many local ordinances permit up to 1,200 sf detached.
- Height
- State law protects 16 to 18 ft for a detached ADU, and up to 25 ft when attached to a two-story house. Four-foot side and rear setbacks are the statutory floor.
- Parking
- No replacement parking may be required for a garage conversion, and no ADU parking at all within half a mile of transit.
- Oak trees
- Placer and El Dorado county oak preservation ordinances drive placement more than any other single factor on wooded parcels.
- Utilities & septic
- A separate water meter, panel upgrade or septic expansion adds real money and weeks of coordination. This is the line people forget.
Rules change. Treat this as orientation, not a code citation. We confirm current requirements against your specific lot before design begins.
Frequently asked
What drives the cost of an ADU in the Sacramento area?
Unit type and utilities. A garage conversion reuses a slab and structure, a detached unit needs its own foundation and service, and a two-story garage apartment adds structure and stair. Septic capacity, a water meter and a panel upgrade move the number more than finishes do. We price your parcel line by line before design starts.
How long does it take to build an ADU?
Roughly 2 to 4 months for design and permitting, since ADUs get ministerial review with a 60 day decision clock, then 6 to 9 months of construction for a detached unit.
Can I build an ADU on my lot?
Almost certainly yes. California law requires cities and counties to approve a compliant ADU on residential lots, subject to objective standards for size, setbacks, height and utilities. We run a feasibility review, including septic capacity, before any design work begins.
Does an ADU add value to a home here?
Generally yes, both as rentable income and as appraised value, and California now allows an ADU to be sold separately in some cases. The gain depends on the neighborhood and on the unit being permitted and finished to the same standard as the main house.