ADU or addition? The 500 sq ft question
Same square footage, different rules, different house. How to decide, with the numbers that matter in San Jose.
Most people who call about an addition have thought about an ADU, and most people who call about an ADU have thought about an addition. They are often the same 500 sq ft. The difference is what the rules let you do with it, and what it does to the house you already have.
What each one is
An addition is more house: a room or rooms attached to the one you have, sharing its kitchen, its entrance and its address. An ADU (accessory dwelling unit) is a second, smaller home on the same lot, with its own kitchen, bath, entrance and, usually, address. It can be detached in the yard, attached to the house, or made from a converted garage.
The rules, side by side
| Question | Addition | ADU |
|---|---|---|
| Planning hearing | No, if within setbacks and lot coverage | No, under 800 sq ft and 16 ft (state law) |
| Side and rear setback | 5 ft side, 20 ft rear typical | 4 ft side and rear |
| Lot coverage | Counts against the 40 percent limit | Exempt up to 800 sq ft |
| Kitchen required | No | Yes, full |
| Separate address and meter | No | Usually yes |
| Can be rented separately | No | Yes, 30 days or longer |
| Fire separation | No | Yes, if attached or within 5 ft |
| Permit timeline | 9 – 13 wks | 6 – 9 wks |
| Time on site | 14 – 18 wks | 16 – 24 wks |
Why the number changes at 500 sq ft
Below about 500 sq ft an ADU carries a lot of fixed cost for its size: a full kitchen, a full bath, a water heater, its own electrical sub-panel, utility trenching back to the street, fire separation and a separate inspection sequence. Those items are the same for a 400 sq ft unit as for a 750 sq ft one. An addition of the same size has none of them if it is a bedroom or family room, and only the bath if it is a suite.
Above 500 sq ft the gap narrows, because the kitchen and bath become a smaller share of the whole, and the ADU starts to earn its keep as a separate dwelling. Our rough rule: under 500 sq ft, an addition is the better-value room; over 500, the ADU is worth the extra if you will ever rent it, house a relative in it, or sell the property on the strength of it.
Under 500 sq ft, build the room. Over 500, build the house.
Choose an addition when
- You want the space to feel like part of the house: a bigger kitchen, a family room, a primary suite.
- Nobody will ever live in it separately.
- Your lot has coverage to spare and you want the shortest build.
- You do not want a second kitchen to maintain.
Choose an ADU when
- A parent, an adult child or a tenant will live in it.
- Your lot is near its coverage limit; the 800 sq ft exemption is the only way to add space.
- You want the yard, not the house, to carry the new building.
- You are thinking about what the property is worth with a second address on it.
The garage question
A garage conversion is the fastest ADU in San Jose: slab, roof and walls are already there, and state law does not require you to replace the parking. Time on site is typically 10 to 14 weeks. The limits are the ones you can see: a standard two-car garage is about 400 sq ft, the slab is rarely insulated, and the door opening becomes a wall with a window in it. It is a good small unit and a poor large one.
What we check on the first visit
Setbacks from the fence, the distance to the main house, where the sewer lateral runs, what the panel will carry, and whether a protected tree is in the way. With those five answers we can tell you on the day which of the two the lot wants, and roughly how many weeks each would take.