1. Underestimating the total budget
Homeowners often budget the construction number and forget design fees, permit fees, utility upgrades, landscaping restoration, and contingency. The fix: budget 15–20% above the construction estimate to capture true all-in cost.
2. Choosing the cheapest builder
The lowest bid almost always wins by underestimating scope or by skimping on quality. Mid-project change orders and rework usually cost more than the savings. The fix: choose builders by reputation, references, and detailed scope clarity — not by the bottom-line number.
3. Skipping permits
Unpermitted ADUs are nearly impossible to insure, can't be financed at refinance time, get heavily discounted at resale, and put you at risk of city enforcement. The fix: permit everything from day one. The cost is small relative to the value protection.
4. Designing for yourself instead of the tenant market
If your primary goal is rental income, the unit should match what tenants in your neighborhood actually pay top dollar for. The fix: research comparable rentals in your area before finalizing layout and finishes.
5. Trying to manage the project yourself
Owner-managed ADU projects almost always run longer and cost more than design-build projects with a single accountable team. The fix: hire a team that handles design, permits, and construction under one roof.