500 sq ft ADU cost in San Diego, and why 499 sq ft is cheaper

500 sq ft ADU cost in San Diego

The clearest public number at this size comes from Snap ADU's published San Diego prices, updated March 2026. Its 500 sq ft detached unit is a one-bedroom, one-bath:

The all-in figure leaves out what Snap ADU marks as variable: extra sitework such as grading, retaining walls or demolition, extra utility work, finish upgrades and city fees. Snap ADU's sitework guide prices a separate SDG&E meter at $10,500 to $15,000 and a sewer pump at about $5,000. Those don't shrink with the unit, which is why 500 sq ft costs more per foot than any larger size on the list.

For other sizes and types, see what an ADU costs in San Diego, or plug your own lot into the ADU cost calculator.

499 vs 500 sq ft: the school-fee line

State law exempts an ADU under 500 square feet from school fees. A unit of exactly 500 isn't under 500, so it pays. In San Diego Unified, the rate is $5.38 per square foot, or $2,690 on a 500 sq ft ADU. Other districts set their own rates.

School fees aren't the only step. Snap ADU's February 2026 fee table for the City of San Diego shows plan check and building permit charges moving to a higher rate above 500 sq ft:

The table is a builder's estimate, not an official city schedule, and it uses an older school fee rate than San Diego Unified's current $5.38. Still, the pattern is clear: of the $14,404 difference, almost all comes from the three charges that change at 500 sq ft. Between 500 and 749 sq ft, the table's lines are flat except the school fee, so most of that jump lands as soon as you cross the line, not gradually up to 749.

Impact fees are a separate line, and 500 sq ft is well under it. State law bars impact fees on any ADU of 750 sq ft or less, and the City of San Diego exempts the first two ADUs on a lot at any size.

How big is a 500 square foot ADU?

Five hundred square feet is a 20-by-25-foot rectangle. That's a bit bigger than a two-car garage, which This Old House puts at about 400 to 440 square feet. It's also slightly larger than a typical studio apartment: RentCafe's size analysis puts the average U.S. studio at 460 sq ft and one-bedroom at 735. Since January 1, 2026, state ADU size limits use interior livable space, so the 500 is measured inside the walls.

If you're still deciding between this and something smaller, our 400 sq ft ADU guide covers the studio end, and the small ADU guide covers why people build small at all.

500 sq ft ADU floor plans

Around 500 sq ft, a bedroom with a door stops being a squeeze. Escondido's free Plan 1 is 484 sq ft and is drawn to work as either a studio or a one-bedroom, one-bath unit, so it fits under the school-fee line. Snap ADU builds its 500 sq ft price around a one-bedroom, one-bath.

A sample 500 sq ft one-bedroom layout, as designers commonly arrange it:

• Living and kitchen: the front half, with the kitchen along one wall and a small dining spot at a counter or table.
• Bathroom: in the middle of the plan, sharing a plumbing wall with the kitchen and acting as a sound buffer.
• Bedroom: at the back, with room for a queen bed, nightstands and a closet.
• Laundry: a stacked washer and dryer in a hall closet next to the bathroom.

As a studio, the same 500 sq ft gives you one large room with space for a sofa, a bed and a table for two. For layouts at every size, see ADU floor plans.

Other rules at 500 sq ft

Sources: Government Code § 66333 for junior ADUs, § 66321 for the 800 sq ft guarantee and San Diego's ADU code for the local range. From January 1, 2027, SB 1117 limits impact fees on larger ADUs to the area above 750 sq ft. It doesn't change the 500 sq ft school-fee line. More in California ADU laws.

500 sq ft ADU vs cheaper options

If $300,000 is more than you planned, the same 500 sq ft can come from space you already have. Lars Remodel's October 2026 San Diego planning ranges:

A junior ADU can share the main home's bathroom and needs only an efficiency kitchen. See junior ADU rules and the garage conversion ADU guide.

What a 500 sq ft ADU rents for

A 500 sq ft unit can be listed as a studio or a one-bedroom, and that label matters. HUD's 2026 fair market rents for San Diego County are $2,288 a month for a studio and $2,459 for a one-bedroom, a $171 gap. Fair market rents describe modest units with utilities included, so use them as a floor for comparison, not a forecast.

Our guide to ADU rental income in San Diego works through payback at this size. When you're ready for bids, compare the best ADU builders in San Diego and check a contractor's license before you sign.

Common questions