An ADU is one of the highest-return projects a San Diego homeowner can build — but a few avoidable mistakes quietly add tens of thousands to the final bill. Here are the three that cost homeowners the most, and how to sidestep each one.
1. Building before you check your utility lines
The cheapest time to find a sewer or water line is before the slab is poured. Discover it afterward and relocating it can easily run $10,000–$20,000 on its own. Have your contractor locate and map your utilities during the design phase — not once the excavator is already on site.
2. Taking the cheapest bid
When one bid comes in far below the rest, it's rarely because that contractor found a magic discount — it's because something got left out. Thinner materials, a smaller crew, permits quietly omitted. You end up paying for it twice: once for the low bid, and again to fix what it skipped. Compare bids line by line, not by the bottom number.
3. Skipping the permit math
Permitting is the cost homeowners underestimate most. The building permit itself is only a few thousand dollars — but all-in, once you add plan check, school fees, water/sewer, and SDG&E, San Diego ADU permitting commonly runs $14,000–$22,000. Budget it up front or it eats into your build. One bright spot: ADUs under 750 sq ft are exempt from development impact fees under AB 881 — a $5,000–$15,000 savings — and units under 500 sq ft get full school-fee waivers.