Cost breakdown for a typical 800 sq ft detached ADU
| Cost category | Low | High | Notes |
|---|---|---|---|
| Construction (800 sq ft) | $200,000 | $400,000 | $250-$500 per sq ft |
| Building permit | $3,000 | $5,000 | Based on project valuation |
| Park impact fee | $4,500 | $4,500 | Flat fee, Madison Parks |
| Architect / design | $8,000 | $15,000 | Plans, energy compliance |
| Site work | $15,000 | $30,000 | Utilities, excavation, grading |
| Total | ~$230,000 | ~$455,000 | $288-$569 per sq ft all-in |
Cost by ADU type
- Detached new construction — $250-$500/sq ft. Most expensive but gives you the most design flexibility and privacy.
- Attached addition — $200-$400/sq ft. Saves on foundation and some utilities if connected to existing house.
- Basement conversion — $150-$300/sq ft. Structure already exists; costs are finish work, plumbing, egress windows, and separate entrance.
- Garage conversion — $100-$250/sq ft. Cheapest option — existing structure, but may need foundation/insulation upgrades to meet building code for habitable space.
Factors that affect your Madison ADU cost
- Site access & excavation: tight lots, sloped lots, or limited access for construction equipment add $5K-$20K.
- Utility connections: if water/sewer/electrical need new runs to the ADU, add $10K-$30K. Madison allows shared utilities with the primary dwelling if a deed restriction is added.
- Foundation type: slab is cheapest, crawl space is moderate, full basement adds $30K-$60K but doubles your living area.
- Finishes: builder-grade vs mid-range vs high-end can add $50K-$100K on a small ADU.
- Season: winter construction in Madison is more expensive due to frost protection, heating costs.
- Contractor availability: Madison's housing market is tight. Expect to bid 3+ contractors and plan 3-6 months out.
Financing options
Most Madison ADU owners use one of four financing approaches:
HELOC (Home Equity Line)
~7.5% rates. Draw as needed. Requires 15-20% equity.
Construction-to-Permanent Loan
~8% rates, 20% down, converts to 30-year mortgage.
Cash-Out Refinance
Replace primary mortgage with larger one. Attractive when rates fall.
Cash / Savings
No interest cost. Best ROI but ties up capital.
Note:Before April 2024, Madison's owner-occupancy requirement made many lenders decline ADU loans. With that requirement removed, financing is significantly easier — including for investors building ADUs on rental properties.
Rental income and ROI
Based on HUD Small Area Fair Market Rent (SAFMR) FY2026 data for Madison, typical ADU rents are:
- Studio: $1,120-$1,570/month depending on ZIP
- 1-bedroom: $1,310-$1,840/month
- 2-bedroom: $1,500-$2,100/month
- Premium ZIPs (53715, 53703): command 20-30% higher rents due to proximity to downtown and UW campus
Typical ROI math for a $350,000 detached 2-bedroom ADU renting at $1,800/month:
- Annual gross rent: $21,600
- Expenses (30%): $6,480 (taxes, insurance, maintenance, vacancy, management)
- Net operating income: $15,120/year
- Cap rate: ~4.3%
- Simple payback: ~23 years cash, shorter with financing and appreciation
- Long-term: significant equity building + property value increase
See our Madison ADU guide for the complete rules, setbacks, and permit process. Or sign up to run a property-specific ADU analysis with exact cost estimates, projected rent from HUD data, and a site plan — all pre-filled from Madison assessor data.
Frequently Asked Questions
What is the cost per square foot for a Madison ADU?+
Detached new-construction ADUs cost $250 to $500 per square foot in Madison in 2025-2026. Attached ADUs and conversions are often cheaper, $175-$350 per sq ft. Source: RenoFi, Critchfield Construction, and Madison-area contractor data.
What does a typical 800 sq ft Madison ADU cost all-in?+
Approximately $225,000 to $450,000 all-in. This includes construction ($200K-$400K), permits (~$5,000), the $4,500 park impact fee, design/architect fees ($8K-$15K), and site work including utility connections ($15K-$30K).
What is Madison's park impact fee for ADUs?+
Approximately $4,500 per ADU, paid at building permit issuance. Verify the current rate with Madison Parks Department.
How much can I earn renting out a Madison ADU?+
Based on HUD Small Area Fair Market Rent (SAFMR) FY2026 data for Madison: studios rent for ~$1,200-$1,500/mo, 1-bedroom ADUs $1,400-$1,800/mo, and 2-bedroom ADUs $1,600-$2,000/mo depending on ZIP code. Higher-income ZIPs like 53715 (near downtown/campus) command premium rents.
What is the ROI on a Madison ADU?+
A typical $350K ADU renting at $1,800/month generates ~$21,600/year gross, or ~$15,000/year net after 30% expenses. That's a ~4.3% cap rate and a ~23-year payback if paid in cash. Appreciation and rent growth improve returns significantly over time. Many investors use HELOC or construction financing.
How do I finance a Madison ADU?+
Common options: (1) HELOC on existing home equity — ~7.5% rates, fast approval; (2) Construction-to-permanent loan — ~8% rates, 30-year amortization after construction; (3) Cash-out refinance if mortgage rates have dropped; (4) Personal savings. The 2024 removal of owner-occupancy requirements opened up more lending options for investors.
Is the cost of an ADU higher for smaller units?+
Yes, on a per-square-foot basis. Fixed costs like permits, utility hookups, and foundation are similar regardless of size, so a 400 sq ft studio can cost $400-$600/sq ft while a 1,000 sq ft unit comes in at $250-$400/sq ft.
Can I save money with a prefab or modular ADU?+
Potentially — prefab/modular ADUs can reduce construction time and sometimes total cost by 10-20%, but savings vary. Madison's site work, foundation, utility connections, and permits are the same as stick-built. Factor in crane fees and delivery. Companies like Abodu, Villa, and Boxabl serve the Madison market.