Check the original sources and current program guidance before making a property-specific decision.
What does an 800 sq ft ADU cost in the model?
ADU.app model v1.1 estimates about $345,000 for a refined detached 800 sq ft ADU on a moderate site, with an early planning range of roughly $293,000–$397,000.
The calculation uses published model assumptions, 14% for design and permits, 8% contingency, and a plus-or-minus 15% early range. It intentionally exposes the math so homeowners can challenge each assumption.
The result is not a contractor quote or California average. A difficult slope, long utility runs, premium materials, service upgrades, fire requirements, or restricted access can move the project outside the range.
| Approach | Illustrative total | Planning range | Effective total / sq ft |
|---|---|---|---|
| Garage conversion | $234,000 | $199,000–$269,000 | $293 |
| Attached addition | $296,000 | $252,000–$341,000 | $371 |
| Detached new build | $345,000 | $293,000–$397,000 | $432 |
| Prefab / modular | $317,000 | $270,000–$365,000 | $397 |
Why can the effective rate be lower than a 500 sq ft ADU?
The model’s fixed allowances and core systems are distributed across 800 square feet instead of 500, reducing the effective all-in rate even though total cost rises.
In the detached examples, the model produces about $482 per square foot at 500 square feet and about $432 per square foot at 800 square feet. That difference is a mathematical result of the assumptions, not a promise of scale savings on every site.
A larger footprint may also require more grading, roof area, foundation, windows, finishes, heating and cooling, or landscape repair. Test the actual site rather than extrapolating from area alone.
What belongs in an 800 sq ft program?
Start with occupant needs, then test bedrooms, bathroom count, storage, laundry, mechanical space, circulation, daylight, privacy, and outdoor access within the verified envelope.
An 800-square-foot plan can sometimes support two bedrooms, but bedrooms should not be squeezed in merely for a listing label. Door swings, closets, egress, sound separation, furniture, and shared living quality matter.
- Draw real furniture and storage before approving the room count.
- Confirm allowable floor area, height, setbacks, and lot coverage.
- Coordinate mechanical and utility space early.
- Price one-bath and two-bath options as separate scopes.
- Compare complete proposals with written allowances and exclusions.
How do you move from model to bid?
Replace model assumptions with a survey, feasibility findings, defined drawings, a written specification, agency information, and comparable contractor proposals.
Early estimates are most useful for choosing which option deserves deeper study. A construction commitment should rely on a much more complete design and property record.
Ask bidders to identify unknowns and exclusions. If one proposal omits utilities, appliances, permits, or site restoration, adjust the comparison before deciding which number is lower.
COMMON QUESTIONS
Frequently asked questions
Direct answers to related questions, with important limits and verification steps kept visible.
Can an 800 sq ft ADU have two bedrooms?+
It may, depending on the plan, applicable standards, code, egress, storage, and intended occupants. Test livability instead of relying only on bedroom count.
Does an extra bathroom add much cost?+
It can. Plumbing, waterproofing, fixtures, ventilation, finishes, utility capacity, and labor all add scope; request a priced alternate.
Is the example a guaranteed range?+
No. It is an early model range. Actual bids can be lower or higher based on the property, location, design, timing, and contract scope.
OFFICIAL AND PRIMARY SOURCES
Sources and methodology
Rules and program guidance can change. Review the latest official material and confirm property-specific requirements with the responsible agency and qualified professionals.
This article provides general educational information and does not replace property-specific legal, financial, tax, design, engineering, appraisal, lending, or construction advice. ADU.app’s editorial team writes the article; the research-review role checks citations and calculation consistency. Last updated September 8, 2026. Confirm current requirements with official sources.