ADU Construction & Room Additions in San Jose, CA

Nobody is being convinced of anything. Either the drawings demonstrate that every objective standard is satisfied, or plan check writes corrections and the clock restarts. State law does put deadlines on the agency: a completeness determination on the application, and a decision on a complete application within 60 days under Government Code section 66317. Those deadlines are real, and they are also frequently misunderstood by homeowners, because they run on a complete application. A submittal missing a structural calculation or a Title 24 compliance report is not complete, and the clock has not started.

Getting the package right the first time is worth considerably more than any promise about speed. Four build types cover almost everything homeowners ask for, and they differ mainly in how much of the existing house they disturb. A detached ADU is a new structure in the yard with its own foundation, its own roof and its own utility connections. It gives the most separation, which is why it suits rental use or a family member who wants a real front door, and it is the most construction because nothing already exists.

An attached ADU shares a wall with the house. That saves one wall assembly and shortens the utility runs, but it means cutting into an existing structure and matching what is already there. A garage conversion reuses a shell that is already standing, which reduces framing and roofing dramatically, but the existing slab, headers and roof structure all have to be evaluated rather than assumed, and slabs poured for a car were rarely poured for a dwelling.

A junior ADU is a unit created entirely within the walls of an existing or proposed single-family residence, including an attached garage. State law caps a JADU at 500 square feet, allows only one per single-family lot, and permits it to share a bathroom with the main house. Note that where a JADU does share a bathroom, an owner-occupancy condition attaches to either the JADU or the primary residence, with some exceptions. That condition is worth understanding before you choose the JADU route for a pure rental play.

The physical build follows an order that cannot be shuffled. Site work and utility locating come first, then forming and pouring the foundation. Where a soils report calls for it, that foundation may be deepened, thickened or switched to a pier and grade beam arrangement, and that determination belongs to the engineer rather than to anyone's estimate. Framing goes up next, walls then roof, and in this seismic region the shear walls, hold-downs and anchor bolts in the engineered plan set are not optional detail.

They are the reason the structure is permitted. Once the shell is dried in, the electrical, plumbing and mechanical rough-ins go in together, because they compete for the same wall and ceiling cavities and the order matters. Rough inspections follow. Only after those sign off does insulation go in and drywall go up, since drywall hides everything an inspector needs to see. Then flooring, cabinets, doors, windows, fixtures and trim, and finally the inspections that close the permit.

Utilities are where ADU projects most often turn out to be bigger than they looked from the driveway. A second dwelling adds load the original house was never sized for. An older panel may not have the capacity or the physical spaces for the new circuits, and where that is the case the panel work becomes part of the project rather than a footnote to it. The water service may need upsizing. The sewer lateral has to actually accept the additional fixture units, and on older properties the condition of that lateral is a genuine unknown until somebody looks.

None of this is exotic, but all of it is easier to price and schedule when it is identified during design rather than discovered after the foundation is poured. Room additions run on completely different rails, and conflating them with ADUs is the most common and most expensive misunderstanding in this category. An addition expands the existing dwelling. It gets no ministerial path, no protected four-foot setback and no statutory size floor. It is subject to the ordinary zoning that governs the main house, which means setbacks, lot coverage, floor area limits and, in some neighborhoods, design review.

Structurally it is more delicate work than a detached ADU, because you are operating on a building that is already standing. The new foundation has to tie into the old one without differential settlement between them, and the new finished floor has to land at the same height as the existing floor, which sounds obvious and is routinely missed on houses that have moved over decades. The new roof has to tie into the existing roof at a pitch and a valley that sheds water rather than collecting it.

If the addition means removing a load-bearing wall, that load has to be picked up by a beam, carried down through posts, and landed on footings sized for the point loads, all of it engineered and drawn before demolition rather than after. New conditioned floor area triggers Title 24 compliance. The existing furnace and air conditioner may not have the capacity for the added volume, and the duct layout usually needs reworking rather than extending.

A new bedroom needs a compliant egress opening. Smoke and carbon monoxide alarms typically have to be brought up to current standard throughout the house, not only in the new rooms. On an older home, matching stucco, siding, roofing and window profiles so the addition does not announce itself is its own separate problem, and it is the one most likely to be judged by everyone who visits.