Almost every ADU budget conversation I have starts with a size, and almost none of them start with a type. People say "I want about 800 square feet" long before they know whether that is a conversion or a new build, which is backwards, because the type moves the number more than the size does.
So this section prices it the way people actually ask. Pick your size, see all five build types side by side at that size, then read what specifically changes there: which types drop out, which fee thresholds you cross, and what your city index does to the total.
If you already know your type, the ADU types hub is the better door. If you know your city, start from the cost pillar instead.
Every size, every type, side by side
National point estimates at a standard finish, before any city adjustment. Blank cells are sizes the type cannot legally or realistically be built at.
| Size | Detached ADU | Attached ADU | Garage Conversion ADU | Junior ADU | Above-Garage ADU | All-in range |
|---|---|---|---|---|---|---|
| 600 sq ft ADUpriced at 600 sq ft | $180,000 | $142,800 | $109,800 | Not at this size | $150,000 | $82,350–$225,000 |
| 800 sq ft ADUpriced at 800 sq ft | $240,000 | $190,400 | $146,400 | Not at this size | $200,000 | $109,800–$300,000 |
| 1,000 sq ft ADUpriced at 1,000 sq ft | $300,000 | $238,000 | $183,000 | Not at this size | $250,000 | $137,250–$375,000 |
| 1,200 sq ft ADUpriced at 1,200 sq ft | $360,000 | $285,600 | $219,600 | Not at this size | $300,000 | $164,700–$450,000 |
| 1 bedroom ADU400 sq ft to 600 sq ft | $150,000 | $119,000 | $91,500 | $59,850 | $125,000 | $39,900–$225,000 |
| 2 bedroom ADU700 sq ft to 1,000 sq ft | $255,000 | $202,300 | $155,550 | Not at this size | $212,500 | $96,075–$375,000 |
The junior ADU column is capped at 500 sq ft of interior livable space by California Government Code section 66313, which is why it empties out above the one bedroom row. Bedroom rows are priced across the size band shown under them.
Browse ADU costs by size
600 sq ft ADU cost
The cheapest size that still lives like a real one bedroom, and it stays under the fee line.
$82,350–$225,000800 sq ft ADU cost
The most requested size, and the first one that pays impact fees in California.
$109,800–$300,0001,000 sq ft ADU cost
The largest unit California has to let you build, and the point where type choice stops mattering.
$137,250–$375,0001,200 sq ft ADU cost
Past every state size floor, so the ordinance decides whether you can build it at all.
$164,700–$450,0001 bedroom ADU cost
Priced at 400 to 600 sq ft. The only size where a junior ADU is still on the table.
$39,900–$225,0002 bedroom ADU cost
Priced at 700 to 1,000 sq ft, the range that straddles California fee and size thresholds.
$96,075–$375,000