Every ADU conversation starts the same way: standing in the backyard, squinting at the fence line, asking whether the thing will even fit. More often than you'd guess, it will. California has spent years sanding down the local rules that used to kill these projects, and most San Jose single-family lots now have room for one. Here's the feasibility check we run before anyone draws a line.
- Detached ADUs need a 4-foot setback from rear and side property lines.
- State law guarantees you can build an ADU of at least 800 sq ft and up to 16 ft tall at a 4-ft setback, even where local rules are stricter.
- Size caps: up to 1,000 sq ft on lots under 9,000 sq ft, up to 1,200 sq ft on larger lots.
- Rear-yard structures are limited to about 40% rear-yard coverage (or 800 sq ft, whichever is greater), which often sets the real limit on a small lot.
The setbacks that shape your footprint
A detached ADU in San Jose generally needs a 4-foot setback from the rear and side property lines. Just as important, California law provides a baseline: regardless of local standards, you're allowed an ADU of at least 800 square feet, up to 16 feet in height, meeting a 4-foot side and rear setback. That state floor is why so many lots that look too tight on paper still pencil out. Front setbacks follow your zone's existing requirement.
Size limits and the rear-yard coverage rule
Two numbers control how big you can go:
- Maximum unit size: up to 1,000 sq ft for a detached ADU on a lot under 9,000 sq ft, and up to 1,200 sq ft on a lot of 9,000 sq ft or more. JADUs are capped at 500 sq ft inside the existing home.
- Rear-yard coverage: structures in the rear yard generally can't cover more than 40% of the rear yard, or 800 sq ft, whichever is greater. On a modest lot this, not the unit cap, is usually what limits the size.
So a big lot may allow the full 1,200 sq ft, while a small backyard might top out closer to the 800 sq ft coverage floor. Both still clear the 800 sq ft state guarantee.
Parking: often not the obstacle it seems
San Jose typically asks for one off-street parking space for an ADU, but that requirement is waived if your property is within a half-mile walk of a transit stop, which covers a large share of the city. A garage conversion also doesn't have to replace the parking the garage removes. In practice, parking kills far fewer ADU plans than people walk in expecting.
Your quick feasibility checklist
- Lot size: note whether you're under or over 9,000 sq ft; it sets your max unit size.
- Buildable rear-yard area: measure the open yard, then apply the 4-ft setbacks and the 40%-or-800-sq-ft coverage limit.
- Existing structures: count garages, sheds, and patios that already eat into rear-yard coverage.
- Utilities: locate sewer, water, and the electrical panel; distance and capacity affect cost more than feasibility.
- Transit proximity: within a half-mile of transit? Parking likely waived.
- Trees and easements: protected trees and utility easements can shift placement.
If those line up, your lot almost certainly supports an ADU. The next questions are budget and type. Our ADU cost guide and ADU vs. garage conversion comparison walk through those, and the permits overview covers approvals.
Why a site visit beats a spreadsheet
A parcel map gives you dimensions and not much else. It won't show the slope, the shed already eating into your rear-yard coverage, where the sewer lateral runs, or the tree you're not allowed to touch. We walk the property, check it against current San Jose standards, and tell you what actually fits before you spend a dollar on design.