There are four common ways to build an ADU: a markup general contractor, a design-build firm, a prefab/modular company, and an independent owner’s-representative project manager. They differ most in how the builder is paid and how much of the work is competitively bid — which is what ultimately drives your cost.
The four models, compared
| Model | How they’re paid | Cost pressure | Best for |
|---|---|---|---|
| Markup general contractor | Margin on every trade | Higher — markup baked in | Small, simple jobs |
| Design-build firm | One bundled fee for design + build | Convenient, but limited bidding | Buyers who want one vendor end-to-end |
| Prefab / modular | Fixed product price + site work | Low on standard units; site work varies | Standard layouts on simple sites |
| Owner’s-rep project manager | Transparent flat fee | Lower — trades competitively bid | Custom ADUs & new homes; cost control |
How to decide
- Want the lowest price on a standard unit? A prefab/modular ADU can be hard to beat — if a stock layout fits your lot and goals.
- Want one vendor to own design and build? A design-build firm is convenient, though you trade away competitive bidding on the trades.
- Building something custom and want to protect the budget? An owner’s-rep project manager competitively bids each trade and manages the build open-book — which is how Ground Rise Build targets a delivered cost up to 10% below a markup contractor’s bid.
Whichever way you lean, the smartest first move is the same: get more than one number. Bring us your best bid from any of these and we’ll show you, honestly, where we can save. See the owner’s-rep vs. GC deep dive or get a free estimate.
Frequently asked
What types of companies build ADUs in the Bay Area?
Bay Area homeowners typically choose among four models: a markup general contractor, a design-build firm, a prefab or modular ADU company, and an independent owner’s-representative project manager. They differ mainly in how they are paid and how much of the work is competitively bid versus fixed.
Which ADU builder model is cheapest?
No single model is always cheapest, but competitive bidding tends to lower cost. Prefab can save on a simple, standard unit; an owner’s-rep project manager competitively bids each trade and targets a delivered cost up to 10% below a markup general contractor’s bid on a custom build.
Sources
- California Department of Housing and Community Development (HCD) — Accessory Dwelling Units (state ADU law and policy)
- Contractors State License Board (CSLB) — Check a Contractor License (verify any builder’s license before hiring)
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