Planning
Nine things to check before you spend a dollar
Site conditions decide more ADU budgets than design choices do. These are the ones that turn into change orders when nobody looks first.
The short version
- The cost of an ADU is mostly in what your lot makes you do, not in the finishes.
- All nine of these are checkable in an afternoon, most of them for free.
- The expensive surprises are almost always the electrical panel, the sewer connection, and the property line.
- Old unpermitted work is worth raising yourself. There's a state amnesty for units built before 2020.
Every item here has turned into a change order on somebody's job. None of them are hard to check.
- 01
The electrical panel
Go open it and read the label. A 200 amp panel with empty breaker slots is a good day. A 100 amp panel, a full one, or an old fuse box means a service upgrade, and that puts the utility company's schedule on your critical path.
- 02
Sewer or septic
On sewer: where does the line run, and can the new unit reach it downhill? If not, that's a pump and a power circuit. On septic, the system's capacity usually caps your bedroom count, and the county wants a soils evaluation before it'll discuss a floor plan.
- 03
Where the water and gas come in
Meter locations decide trench routes, and trench routes decide what gets dug up and put back. Sixty feet of trench across a mature yard is a real line item.
- 04
Your actual property line
Not the fence. Fences move, get rebuilt in the wrong spot, and are often a foot or more off. Setbacks get measured from the line. If you're building anywhere near the minimum, a survey costs less than moving a wall.
- 05
Easements
A utility or drainage easement is a strip of your land somebody else has the right to use. They're listed on your title report and invisible on a satellite photo. You generally can't build over one, and plan check is an expensive place to find out.
- 06
Trees
Lots of Southern California cities protect oaks, sycamores, and anything over a certain trunk diameter. Protection usually extends well past the canopy to the roots. Removing one, or building near it, can mean its own permit and an arborist's report.
- 07
Fire hazard severity zone
If your parcel is in a high or very high zone, you're into ignition resistant construction: exterior materials, vents, eaves, windows, sometimes sprinklers. It changes the building, not just the paperwork. The state maps are public and searchable by address.
- 08
Old unpermitted work
Converted garages and 1970s additions surface the moment a plan checker looks at your property. Not automatically a problem. AB 2533 created a statewide path to permit units built before January 1, 2020 without fines or impact fees, inspected for health and safety rather than full current code. Raise it yourself rather than have it found.
- 09
Your HOA documents
Read them, but know the law first. Under Civil Code section 4751 an HOA can't prohibit an ADU or junior ADU on a single family lot. It can set reasonable standards. A flat ban is unenforceable, and plenty of CC&Rs still have one sitting in them.
If you'd rather not go crawling around your own foundation, that's fair. It's the first thing we do when we walk a property, and we'll walk you through what turns up.
Sources
- AB 2533 (2024), unpermitted unit amnesty
- California Civil Code section 4751, homeowners associations
- CAL FIRE fire hazard severity zone maps
This is our read of the rules, not legal advice. We write these because they're the questions we get asked, and we keep them current. ADU law changes most Januaries, and every city adds its own rules on top of the state's, so before you spend money on anything here, confirm it with your city or county planning counter. Or call us and we'll confirm it for you.
More guides
ADU law
What California ADU law lets you build
State law sets a floor your city can't go below. Here's what that floor is, what changed in January 2026, and which parts your city still controls.
Choosing
ADU, JADU, or garage conversion: which fits your lot
Four ways to add a unit, what each one costs you in constraints, and the questions that decide it before anyone draws anything.
Process
How an ADU permit actually moves
The six stages between an idea and a building permit, which ones have a legal clock on them, and where the time really goes.
Want this answered for your address?
State law is the floor. What your city allows on top of it is the part that decides your project. Tell us where you are and we'll tell you what fits.