An attached ADU is a new addition joined to your existing home but built as a separate unit, with its own entrance, kitchen and bathroom. It can make sense when the backyard is shallow, when you want to keep open yard space, or when the house sits well on the lot for an extension.
What size limits apply to an attached ADU?
As of October 2026, where a city has no compliant ADU ordinance, the state default caps an attached ADU at 50% of the main home's floor area. Even so, a city's limit must still allow at least 850 sq ft, or 1,000 sq ft with more than one bedroom. San Diego's own code sets the numbers that apply to your zone.
How tall can it be?
As of October 2026, state law requires cities to allow an attached ADU up to 25 feet or the height limit for the main home, whichever is lower. That can allow a second-story unit over part of the house where structure and zoning permit.
When does attached beat detached on a San Diego lot?
- The yard is shallow or steep, but there is room beside or behind the house.
- You want to keep the backyard open for the main household.
- Utility connections are close at hand, which can shorten trenching.
- You want the unit near the house for a family member who may need help.
What does an attached ADU cost?
As of October 2026, San Diego contractors publish ranges from about $160,000 to $350,000 or more. The tie-in to the existing roof and walls, and any upgrades the main house needs to carry the new structure, often decide where a project lands. See the San Diego cost guide.
What has to be checked on the existing house?
We review the main home's foundation, framing and electrical service, since an addition can load all three. Fire separation between the two units is required, and an older house can need some upgrades where the new work connects. Those findings go into your written, itemized quote.