A backyard ADU is the most-requested project on this platform, and the one where good planning saves the most money. Almost every expensive surprise arrives in the first three weeks — in the setbacks, the utilities, or the energy code. Here is the whole sequence, in order.
Start with the envelope your lot allows
Before a single line is drawn, your lot sets the maximum: rear and side setbacks, lot coverage, the height limit, and how close the unit may sit to the main house. California's state ADU exemptions override a great deal of restrictive local zoning, but they do not override everything, and every city applies them slightly differently.
A first-pass feasibility check answers the only question that matters this early: how many square feet can legally stand in your backyard, and where.
Utilities are the line item people forget
Drawings are rarely the expensive part of an ADU — the trench is. Before you commit to a location, find out:
- ✓Whether the existing electrical panel has capacity or needs a service upgrade
- ✓How far the sewer lateral runs, and whether the unit can gravity-feed into it
- ✓Whether the water meter is sized for a second dwelling
- ✓Whether a new gas line is needed, or the unit will be all-electric
- ✓Where fire-department access and the egress path will run
Title-24 and the all-electric question
California's energy code treats an ADU as its own dwelling. In practice most new ADUs now pencil out cheaper all-electric — a heat-pump mini-split and a heat-pump water heater — because it avoids a new gas line and clears Title-24 with less effort. Your architect models the compliance path and the forms go into the permit set.
The real timeline
From a signed scope to a stamped set, a straightforward detached ADU runs about five to seven weeks: a week of as-builts and site measurement, two to three weeks of schematic and design development, then two weeks producing the permit set with structural and energy coordination. Plan check sits on top of that, and it is your jurisdiction's clock rather than your architect's.
“Every week people spend agonising over finishes at the start is a week they wish they had spent on the utility trench.”
— Maya E., AIA · Licensed CA
Settle the envelope, the utilities and the energy path first. The interior layout is the easy part, and it is the only part you can still change late without paying for it twice.