What Actually Drives the Cost of an ADU

The single biggest factor: what already exists

The spectrum runs from converting sound, dry, tall-enough existing space (least new structure) to building a detached unit from the foundation up (most). A basement or garage that's genuinely convertible gives you a head start on shell, foundation, and roof. A detached ADU buys total independence at the cost of every trade starting from zero, excavation, foundation, framing, weatherproofing, utilities.

The cost drivers, in rough order of impact

• ADU type, conversion vs. addition vs. detached new construction, as above.
• Kitchens and bathrooms, the most expensive rooms per square foot anywhere, and an ADU always has both. Fixture and finish tiers move this line a lot.
• Septic, on unsewered lots, whether the existing system supports the added bedrooms or needs an upgrade can swing a budget more than any finish decision.
• Utility connections, trenching power, water, and drainage to a detached unit; panel capacity in the main house; separate vs. shared systems.
• Site conditions, access for equipment, grading, ledge, trees, and how far the unit sits from existing services.
• Structure condition on conversions, a garage slab that must be replaced, framing that needs reinforcement, moisture that needs solving first.
• Systems choices, heating and cooling approach, ventilation, electrical scope, laundry in the unit.
• Finish level, flooring, cabinetry, tile, and trim tiers compound across the whole unit.
• Accessibility features, zero-step entries and curbless showers are most affordable when planned from the start.
• Design efficiency, the state's size cap rewards tight, well-planned layouts; complexity in the footprint and roofline costs more than area.

Where budgets actually go wrong

• Skipping the septic question until after design, the most expensive surprise on rural lots.
• Pricing the construction but not the site work, driveways, walkways, landscape repair, and utility trenches are real line items.
• Comparing bids that describe different projects, one includes appliances and site restoration, another doesn't. A readable scope beats a low number.
• Treating contingency as pessimism, on conversions especially, opened walls tell you things. A planned reserve turns discoveries into decisions instead of crises.
• Designing for resale theories instead of the actual occupant, the unit that serves your real needs is the one that earns its cost.

How to get a number you can trust

1. Start with feasibility on your property, not pricing folklore from other towns.
2. Decide the unit's job (who lives there, for how long) so scope reflects purpose.
3. Get a written scope that names inclusions, allowances, and exclusions in plain English.
4. Ask how discoveries are handled, the process answer tells you more than the price.