Can you build an ADU on your lot? In California, almost every single-family lot qualifies under state law. The real question is how big, where it goes, and, for a prefab ADU, whether a truck and crane can reach the spot. Run through this checklist before you spend money on design.
The 7-point ADU feasibility checklist
| Check | What to look for | Why it matters |
|---|---|---|
| 1. Zoning | Residential or mixed-use zone that allows a home | State ADU law applies in these zones |
| 2. Buildable area | Open yard space after 4 ft side and rear setbacks | Sets your maximum footprint |
| 3. Easements | Utility or access easements on your title report or parcel map | You can’t build on most easements |
| 4. Utilities | Electrical panel size, sewer or septic, water line location | Often the biggest hidden cost |
| 5. Slope and soil | Flat vs. hillside; any known soil issues | Drives foundation cost |
| 6. Hazard zones | Very high fire hazard, flood, coastal zone | May add fire, flood or coastal conditions |
| 7. Delivery access (prefab) | Street width, overhead lines, trees, a crane spot | Decides if prefab is practical and what the set costs |
Utilities: the hidden budget line
- Electrical: many older homes have 100-amp service. An all-electric ADU often needs a panel upgrade or a separate meter. Ask your utility about lead times early.
- Sewer: the ADU usually ties into your existing sewer lateral. The distance and slope from the ADU to the lateral can change the cost a lot.
- Septic: a septic system may need a capacity review by the county health department.
- Water: most ADUs share the main home’s water meter; some cities or owners choose a separate meter.
Delivery access for a prefab ADU
This is where our experience matters most. A factory-built ADU arrives on a truck in one or more sections, then a crane lifts it over the house or fence onto the foundation. Before you commit to prefab, confirm:
- Street access: can a long, wide load reach your curb? Narrow hillside streets, tight turns and cul-de-sacs can rule out larger sections.
- Load width: any load wider than 8 ft 6 in needs a state transportation permit, and wider sections (generally over 12 ft) also need pilot cars and route and time restrictions, which adds cost.
- Crane spot: a crane needs a firm, level setup area, often in the street, and the lift distance to your foundation sets the crane size and cost. Your city may require a street-use permit.
- Overhead lines and trees: power lines over the lift path can stop a set. Plan with your utility in advance.
If access is impossible, panelized or site-built construction may fit better. We’ll tell you that honestly.
Match the ADU to your goal
| Goal | Typical fit |
|---|---|
| Rental income | 1 or 2 bedrooms, separate entry, its own utilities if possible |
| Aging parent or family | Single-level, step-free entry, wider doors and accessible bath |
| Home office or studio | Smaller unit, 500 sq ft or less (interior) to avoid school fees |
| Lowest cost | Garage conversion or junior ADU inside the home |
Frequently asked questions
How big of a lot do I need for an ADU?
State law doesn’t set a minimum lot size for an ADU, and a city can’t use lot size to block an 800 sq ft ADU with 4 ft setbacks. Your real limit is open, buildable space.
Can I put a prefab ADU in a backyard with no side access?
Often yes. Prefab units are usually lifted by crane over the house or fence. Overhead power lines and crane setup space are the deciding factors.
What’s the first thing I should check?
Your electrical panel and sewer location. They are the most common surprise costs.
Send us your address and a few photos of the yard and street, and we’ll tell you if prefab is practical. Call (800) 806-7485. Next steps: Prefab ADU floor plans and sizes and The California ADU permit process.
By Matt Banes, iModular.com. California modular building specialist with 38 years in the industry. Last updated: October 7, 2026.
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