A Separate Home, Sharing Your Wall

Yes. To qualify as a legal ADU under California law, the unit must have an independent exterior entrance. This is what separates an ADU from a simple addition or in-law suite. The entrance does not need to be dramatic — a standard exterior door off a private patio or side yard is common — but it must provide direct access from outside without passing through the main home.

Yes. Sharing a wall is required for the unit to be considered "attached." What matters legally is that the ADU is a fully independent dwelling — its own entrance, kitchen, bathroom, and sleeping area — even though it is structurally integrated with the main house. Many attached ADUs share a wall, and some also share a portion of the roofline or floor/ceiling structure.

California requires residential separation assemblies to meet specific Sound Transmission Class (STC) ratings — typically STC 50 for demising walls between dwelling units. We build all attached ADU separation walls to meet or exceed that standard, with staggered studs, double-layer drywall, and acoustic insulation in the cavity. In practice, well-built attached ADUs are quieter than most apartments.

Not always. The two units can share an electrical meter if your panel has sufficient capacity and your jurisdiction allows it, which keeps utility setup costs low. For long-term rentals, though, a separate electrical meter is often required so the tenant can be billed directly. Water and sewer are usually shared unless a separate meter is installed. We design the utility setup around how you plan to use or rent the unit.

Attached ADUs are often the only ADU option available on small or constrained lots. Because they do not require additional separation from the main house and add no new structure to the open yard area in most cases, they can fit within setbacks that would not permit a detached unit. On very narrow lots, a second-story attached ADU over part of the existing home can maximize square footage without any additional ground coverage.

In California, only the value of the new construction is reassessed. Your existing home's Prop 13 tax basis is preserved, and the ADU is added as a separate line item on the assessor's roll. So a $250,000 attached ADU would add roughly $2,500–$3,000 per year to your property tax bill at current rates — while potentially generating $25,000–$40,000 per year in rental income.

Yes, and this is one of the most cost-effective paths available. An attached garage shares a wall with the main house, so converting it into a dwelling unit counts as an attached ADU under California law — with all the same legal rights. The conversion still needs to meet residential standards for insulation, egress, fire separation, and a compliant bathroom and kitchenette, but it avoids the cost of new foundation, framing, and roofing.