ADUs · Additions
ADU or addition — which one do you actually want?
Most people arrive having already picked one. Half of them picked the wrong one for what they're actually trying to do.
An addition grows the house you live in. An ADU is a separate legal dwelling with its own entrance, kitchen and bathroom. Pick based on whether the space needs to be connected or independent — not on which one you assume is cheaper.
The question that usually settles it
Does the space need its own front door? If you want rental income, or genuinely independent space for a parent or adult child, you want an ADU — an addition cannot legally be rented as a separate dwelling. If you need a bigger kitchen, another bedroom off the hallway, or a family room that connects to the house you already live in, you want an addition.
On cost, they're closer than people think
| ADU | Addition | |
|---|---|---|
| Fixed soft costs before construction | ~$13,500–$18,500 — plans, survey, soils report | Lower, but engineering is still required |
| Structural work on your existing house | Little to none if detached | Significant — opening walls means re-carrying load |
| Utility runs | Full new service to the unit; distance matters a lot | Extends existing systems, if they have capacity |
| Impact fees | Waived under 750 sq ft under state law | Varies by jurisdiction |
| Rentable as a separate dwelling | Yes | No |
| Adds to the existing floor plan | No | Yes |
Which comes out cheaper depends on your lot and your house, not on a rule. A detached ADU on a flat lot near existing utilities can beat an addition that requires major structural surgery. The reverse is just as common.
What your lot will actually allow
- Usable rear yard. A detached ADU needs somewhere to go, with setbacks respected.
- Access. Whether a machine can reach the build area changes the cost of everything below grade.
- Utility distance. A unit at the back of a deep lot is a longer, more expensive run.
- Panel capacity. Either option may push your existing service past what it can carry.
- Trees and slope. Protections and grading are real constraints, and they are worth checking before anyone draws anything.
One thing that only applies to ADUs
Since January 2025, every California city has to run a pre-approved ADU plan program with a 30-day ministerial approval clock. If a pre-approved design fits your lot, it can take the plan cost off your project and collapse the permitting timeline. There is no equivalent for additions. How AB 1332 works →
Common questions
Is an ADU cheaper than a home addition?
Can I rent out a home addition?
Which adds more value, an ADU or an addition?
Can I build both an ADU and an addition?
Where to go next
Written by the team at Elusive Construction, a licensed California general contractor (B #1112902) building in Santa Clara and San Mateo counties. General information about how residential construction works — not legal advice. For your specific contract, talk to an attorney.
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