Can I build an ADU for under $100k?

It’s uncommon in 2025 once design, permits, utilities, and finishes are included. Smaller garage conversions in lower‑cost markets may come closest.

What adds the most cost per square foot?

Kitchens/baths, complex foundations, long utility trenches, and premium finishes are the biggest drivers.

How long does permitting take?

Often 4–12 weeks depending on the city/county and whether ADU streamlining applies. Check your local planning portal.

ADU Costs in 2025: What to Expect & How to Budget

Q: Are prefab ADUs cheaper than custom builds?

Prefab can reduce build time and change‑order risk. Compare full project totals—including site work and fees—to see which route pencils out on your lot.

Q: How accurate is the calculator?

It provides a planning ballpark using 2025 assumptions. For a firm price, request three comparable builder bids through the Builders page.

Q: Do ADUs increase property taxes?

Usually the assessed value increases by the value of improvements rather than a full reassessment. Consult your local assessor.

See realistic ranges by ADU type, run the live calculator, and learn how site work, utilities, and permits impact your total.

Open Cost Calculator See Cost Ranges

Costs Financing

On this page

TL;DR: Most ADUs land between $180k–$400k depending on type and site work. Get 3 scoped bids (including utilities and permit fees) for an apples‑to‑apples comparison.

Backyard ADU exterior with wood cladding and glass doors

ADU Cost Ranges by Type

Ranges are typical for 2025 in many U.S. metros. Always verify locally for fees/utilities.

Type	Typical Size (sf)	Cost Range	Notes
Detached	500–800	$220k–$400k	Highest site & utility costs; most flexibility
Attached	400–800	$190k–$330k	Shares a wall; moderates exterior costs
Garage Conversion	350–600	$150k–$260k	Structure exists; utility upgrades common
Prefab (module/panelized)	400–800	$180k–$320k	Speed & price certainty; crane access needed for modules

PS: Ask bidders to include site work, utility trenching, and permit/impact fees in their totals.

Live ADU Cost Calculator

Estimated Total: —

Breakdown

Build & finishes —

Site & utilities —

Permits & fees —

Contingency —

This is a planning tool, not a bid. Get 3 builder quotes to confirm scope and timeline.

Defaults are mid‑market assumptions. Adjust sliders for your site and finish level.

Where the Money Goes

60–70% Build & finishes (framing, MEP, interiors)

15–25% Site & utilities (trenching, tie‑ins, grading)

5–10% Permits, fees, & inspections

Contingency (5–15%) helps cover unknowns like utility upgrades or plan revisions.

Real‑World Case Studies

Detached ADU in San Diego with stucco exterior and patio — **750 sf Detached — San Diego**
Total: ~$340k. Permits/design (10 weeks), build (20 weeks). Upgrades: 200A service, mini‑split HVAC.

Garage conversion ADU in Portland with modern interior — **420 sf Garage Conversion — Portland**
Total: ~$195k. Structural upgrades to slab, plumbing reroute, new sewer lateral.

Cost FAQs

Are prefab ADUs cheaper than custom builds?

Often, yes—prefab can reduce build time and change‑order risk. Compare full project totals including site work and fees.

What’s the cheapest ADU type?

Garage conversions typically start lowest because the structure exists, but utility upgrades can change the math.

How accurate is the calculator?

It’s a planning tool using typical 2025 assumptions. For a firm quote, request 3 builder bids.

Do ADUs increase property taxes?

Usually your assessed value goes up by the value of the improvements. Rules vary—check our Regulations guide and local assessor.

Can I finance ADU construction?

Yes—HELOCs, construction loans, and cash‑out refis are common. See our Financing guide.