7 Steps to Legally Build an Accessory Dwelling Unit (ADU) in Madison, WI in 2025

Building an accessory dwelling unit (ADU) on your property in Madison, WI, is an exciting opportunity to add living space, generate rental income, or provide a home for a family member. However, navigating the process requires understanding local laws, financing options, and construction practices to ensure compliance with regulations and a smooth project timeline.

This guide will walk you through the steps to legally build an ADU in Madison, providing insights into zoning requirements, financing strategies, design and construction considerations, and the importance of working with experienced professionals like Boundless Tiny Homes.

What is an ADU?

An Accessory Dwelling Unit (ADU) is a secondary living space built on a property with an existing primary residence. These units can take various forms, such as detached backyard cottages, garage apartments, or basement conversions, and they offer flexibility for homeowners seeking multigenerational living solutions or additional income opportunities.

In Madison, ADUs are a solution for the city’s growing housing needs, offering a sustainable and affordable option to expand housing stock while maintaining neighborhood character. Whether you’re looking to build a detached or attached unit, understanding the process is essential.

Step 1: Understand Local Zoning and Regulations

Madison has specific zoning laws governing ADU construction. According to the city’s Zoning Code, here are key guidelines to keep in mind:

• Zoning Districts: ADUs are allowed in most residential zoning districts (e.g., SR-C1, SR-C2, TR-C1, TR-C2). Verify your zoning classification using Madison's online zoning map or contact the city’s Zoning Division.

• Lot Size: Your property must meet minimum lot size requirements. For single-family lots, the typical minimum is 5,000 square feet.

• Setbacks: ADUs must maintain at least 4 feet of side and rear yard setbacks and adhere to any front yard setback rules for detached units.

• Height Restrictions: Detached ADUs cannot exceed 25 feet in height. Attached ADUs must align with the existing home’s height.

• Unit Size: ADUs in Madison are limited to 900 square feet or no more than 75% of the size of the primary residence, whichever is smaller.

• Owner Occupancy: The property owner must live in either the primary residence or the ADU.

• Parking Requirements: Additional off-street parking may be required unless waived by city officials.

Before proceeding, consult Madison’s zoning office or work with Boundless Tiny Homes to ensure your property qualifies for an ADU.

Step 2: Plan Your ADU's Purpose

Determine how you plan to use your ADU. Will it serve as a home for aging parents, a guest suite, or a rental unit? Your intended use will influence the design, size, and amenities.

For example:

• Rental Income: Include features like a full kitchen, laundry hookups, and private entrances to attract tenants.

• Family Use: Focus on accessibility features such as no-step entrances, wider doorways, and single-level living for elderly occupants.

Step 3: Secure Financing

Building an ADU is a significant investment, but financing options make it accessible. Common funding sources include:

• Home Equity Line of Credit (HELOC): Flexible for homeowners with substantial equity.

• Construction Loans: Tailored for new builds and converted to a mortgage upon project completion.

• Cash-Out Refinance: Access your home’s equity by refinancing your existing mortgage.

• City of Madison Programs: Check for grants or low-interest loans aimed at increasing affordable housing.

Boundless Tiny Homes can connect you with trusted lenders familiar with ADU projects in the Madison area.

Step 4: Design and Permitting

Work with Boundless Tiny Homes to design an ADU that meets your needs and complies with Madison's zoning and building codes. Our team handles:

• Drafting architectural plans.

• Ensuring compliance with Wisconsin Uniform Dwelling Code (UDC).

• Navigating the city’s permitting process, which typically takes 4-8 weeks.

Common permit requirements include a site plan, construction drawings, and utility connections for water, sewer, and electricity.

Step 5: Construction and Inspections

Once permits are approved, construction can begin. Expect the process to take 4-6 months, depending on the complexity of the project. Key stages include:

• Site preparation and foundation work.

• Framing, roofing, and utilities installation.

• Interior finishing and landscaping.

City inspectors will review the project at various milestones to ensure compliance with building codes. Boundless Tiny Homes coordinates these inspections, so you don’t have to.

Step 6: Decide Between DIY or Professional Help

While a DIY approach can save money, it requires significant expertise and time. Most Madison homeowners prefer working with professionals to ensure the project meets legal and quality standards.

Boundless Tiny Homes offers full-service solutions, from design to construction, eliminating the stress of managing contractors and ensuring a seamless experience.

Step 7: Enjoy Your New ADU

Once your ADU is complete, it’s time to enjoy the benefits! Whether you use it as a rental unit, guest space, or a home for loved ones, your ADU will enhance your property’s value and functionality.

Ready to start building? Contact Boundless Tiny Homes for a free property consultation and let us help you create a high-quality ADU tailored to your needs.