Two dwelling units on one lot. A duplex and an ADU are both that — but they’re legally, architecturally, and financially different in ways that matter significantly for what you can build, how you can finance it, and what happens when you sell.
The Legal Definitions
In Madison’s zoning code, a duplex (or two-family dwelling) is a building designed as two dwelling units — typically sharing a common wall or floor-ceiling assembly — with both units integral to the structure. An ADU is a secondary dwelling that is accessory to a primary single-family residence: smaller, subordinate, and existing in relation to the primary home. A duplex is a primary use. An ADU is an accessory use. This is not a semantic difference — it has direct zoning implications.
Where Each Is Permitted in Madison
Duplexes are permitted uses in TR-C2, TR-V2, TR-V3, and mixed residential districts. They are not permitted in base single-family zones (TR-C1, SR-C1) without a variance. ADUs are permitted in virtually all residential zones in Madison, including TR-C1 and SR-C1, as an accessory use on a single-family lot. This is the major practical advantage: if your lot is in a single-family district — which describes the majority of residential Madison — an ADU is what you can build. A duplex requires a different underlying zoning designation that most urban residential lots don’t have.
Financing and Appraisal Differences
Duplexes are classified as 2–4 family investment properties for mortgage purposes. Conventional financing typically requires 15–25% down. Single-family homes with ADUs are typically financed as owner-occupied single-family properties — lower down payments, better rates, easier qualification. For a homeowner building an ADU on an existing single-family lot, the financing path is substantially easier and cheaper than a duplex.
Resale and Buyer Pool
A single-family home with an ADU sells to a broader buyer pool than a duplex. Most buyers using conventional financing can purchase a home with an ADU. A duplex limits the buyer pool to investors or buyers with larger down payments. In Madison’s market, this difference in buyer pool matters at resale. If you’re wondering whether an ADU or duplex is the right path for your specific parcel, a feasibility check will tell you what’s actually permitted and what makes financial sense.