ADU Build Cost Calculator
Estimate the turnkey budget for an accessory dwelling unit by combining construction costs with permits, professional services, utilities, and a contingency buffer.
Planning for Accessory Dwelling Unit Costs
Accessory dwelling units—often abbreviated as ADUs—have emerged as a popular solution for homeowners seeking to create additional living space, accommodate extended family, or generate rental income. Because ADUs generally capitalize on existing land and infrastructure, they can provide a more cost-effective path to housing compared with purchasing a new property. Nevertheless, building even a small backyard cottage or garage conversion involves a constellation of expenses that extend far beyond the visible structure. This calculator assembles the most significant cost components into a single model so that prospective builders can estimate a project budget before breaking ground.
The foundation of any ADU cost estimate is the product of square footage and construction cost per square foot. These per-unit costs vary widely by region, material quality, and design complexity. A simple detached studio built using economy finishes might cost $200 per square foot, while a high-end two-bedroom unit in a seismic zone could exceed $400 per square foot. To capture this relationship, the calculator multiplies the entered size by the chosen rate, producing the core shell-and-finishes cost. In mathematical terms,
Yet this base construction figure is only the starting point. Municipal permitting, impact fees, and utility connection charges can add thousands to the bottom line, especially in jurisdictions that encourage or restrict ADUs through policy levers. Professional services such as architectural design, structural engineering, and soil tests represent another layer of soft costs. The calculator includes individual fields for permits, design, and utility allowances so that users can customize the estimate to their locality. These amounts are summed with the base cost to produce a subtotal:
Construction budgets rarely proceed without surprises. Unforeseen soil conditions, lumber price spikes, or design revisions can all inflate expenses. Builders often include a contingency—typically ten to fifteen percent of the subtotal—to absorb overruns. The calculator applies the contingency percentage multiplicatively:
The grand total is then the subtotal plus the contingency amount. This layered approach mirrors how professional estimators build budgets and offers a transparent window into the components that drive final costs. The calculator outputs this total along with a breakdown of the base and contingency portions to guide decision making.
Typical Cost Components
| Component | Low Range | High Range |
|---|---|---|
| Site work & foundation | $8,000 | $20,000 |
| Framing & exterior | $12,000 | $35,000 |
| Plumbing & electrical | $9,000 | $25,000 |
| Interior finishes | $10,000 | $28,000 |
| Permits & design | $5,000 | $16,000 |
| Utility connections | $3,000 | $12,000 |
Sample Budgets
| Scenario | Size | Total Cost | Notes |
|---|---|---|---|
| Garage conversion | 420 sq ft | $118,000 | Minimal site work, modest finishes |
| Backyard cottage | 640 sq ft | $198,000 | Mid-grade fixtures, new utilities |
| Premium two-bedroom | 820 sq ft | $274,000 | High-end finishes, higher contingency |
Because site conditions and finishes vary dramatically, the ranges and scenarios above should be treated as directional. A hillside lot or historic district may require structural upgrades or design review that push the total higher. Likewise, owners willing to manage parts of the project themselves might trim some soft costs.
For broader planning, compare your ADU budget to financing options with renovation loans or home equity products so that you can confidently decide between different funding paths. When you are ready to explore lending scenarios, try the home construction cost calculator, renovation loan payment calculator, and the rental property cash flow calculator to understand how an ADU could support your long-term goals.