The question comes up in nearly every early-stage ADU conversation: what’s the return at resale? And a follow-up that matters just as much: does the type of ADU — detached or attached — affect how much value it adds? The answer is yes, but not in the way most people expect.
How Appraisers Value ADUs
Residential appraisers use comparable sales to establish value. In Madison and Dane County, ADU-specific comp data is still building. Appraisers often use adjustments or income approach methodology to capture an ADU’s value contribution. This matters because the appraiser’s method affects what a buyer’s lender will lend against. Having documentation of the ADU — permits, certificate of occupancy, lease agreements — makes the appraiser’s job easier and supports a better value conclusion.
Detached ADUs: Higher Value Ceiling
Detached ADUs consistently command the higher end of ADU value contribution in most markets, including Madison. The reasons are practical: complete separation (higher perceived quality for tenants), rental income clarity (appraisers can apply straightforward income approach), and broader buyer appeal (future buyers see a flexible asset). Estimated value contribution for a well-built detached ADU in Madison: $80,000–$150,000+ depending on size, quality, and location. Build cost runs $130,000–$200,000+, so the appraised value may not fully recoup build cost immediately. The long-term picture — rental income plus appreciation — typically makes the math work.
Attached ADUs: Lower Value Ceiling, Lower Build Cost
Attached ADUs are generally valued lower than detached at resale. Less separation, design constraints from working within the existing structure, and appraisal treatment as an in-law suite rather than a true income unit all contribute. However, attached ADUs cost significantly less to build — often $80,000–$130,000 for a garage conversion or addition — which changes the cost-to-value ratio favorably. A lower appraised contribution from a lower-cost build may yield a better return on investment than a higher-appraised detached ADU that cost more to build.
The Madison Market
Properties in Madison with detached ADUs — particularly newer, purpose-built ones in desirable neighborhoods — have sold with meaningful premiums over comparable non-ADU properties. The premium is most defensible when the ADU has an active rental history demonstrating the income stream. If resale value is a key consideration in your ADU decision, the design and quality of the ADU matter. We build ADUs specifically — start with a feasibility check to understand what makes a well-built ADU command maximum value at resale.