The honest answer is: it depends on more variables than most homeowners expect. But you can get to a realistic number quickly if you understand where the money actually goes.
The two cost buckets
Soft costs — design, engineering, permits. Typically the smallest slice of the budget. This includes plan sets, structural engineering, PE stamps where required, and permit fees. This is also where spending a little saves a lot: complete, permit-ready drawings prevent the expensive redesign loops that thin plan sets cause.
Hard costs — construction. The dominant slice. Foundation, framing, roofing, electrical, plumbing, HVAC, insulation, drywall, finishes, and appliances. Within hard costs, the big swing factors are:
- Size — a studio at 400 sq ft costs far less than a 2-bedroom at 750 sq ft, both to build and to permit.
- Site work and utilities — grading, trenching, and running water/sewer/power to a detached structure. This is the most commonly underestimated line item.
- Finishes — the difference between a basic rental-grade unit and a high-end guest suite can move the budget dramatically.
- Market and labor rates — the same plan costs meaningfully different amounts in different metros.
Detached ADU vs garage conversion
A garage conversion reuses an existing foundation, walls, and roof, and the utilities are usually nearby — so it typically comes in well below the cost of new detached construction. Many jurisdictions also make conversions easier to permit. If your goal is rental income or extra living space at the lowest cost, a conversion is often the highest-return path.
How to get a real number instead of a guess
- Use an itemized cost estimator built on plan-specific data rather than generic per-square-foot averages. QuiPlans' construction cost estimator prices by plan and location, trade by trade.
- Confirm feasibility first. A $349 Engineer Review verifies what is buildable on your lot before you buy plans — and the full $349 is credited toward your permit package if you continue.
- Budget the contingency. On any ADU project, plan for surprises in site work and utilities.
The takeaway
Benchmarks are useful for expectations, but your real budget comes from your plan, your lot, and your market. Run the numbers for a specific plan on your property — that number is the one your lender and your contractor will actually work from.
