How to Build an ADU: Step-by-Step Information Guide

Understanding What an ADU Is and Why Homeowners Build Them

An Accessory Dwelling Unit (ADU) is a smaller residential building on the same property as a single-family home. Common types include granny flats, garage conversions, basement apartments, and detached cottages. ADUs typically range from 400 to 1,200 square feet and contain their own kitchen, bathroom, and sleeping area.

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According to the National Association of Home Builders, ADU construction increased by 25% between 2019 and 2023 as homeowners sought additional income sources and multigenerational living arrangements. Homeowners build ADUs for several practical reasons: generating rental income (averaging $800 to $2,500 monthly depending on location), housing adult children or aging parents, creating guest accommodations, or increasing property value. A 2024 survey by the Urban Land Institute found that properties with ADUs sold for 15% to 25% more than comparable properties without them in many markets.

The financial return varies by location and construction method. In California, where ADU regulations are well-established, homeowners reported construction costs ranging from $100,000 to $400,000 depending on whether they converted existing structures or built new ones. In other states, costs may differ based on local labor rates and building code requirements.

Before starting any project, understand that ADU regulations differ significantly by jurisdiction. Some cities prohibit them entirely, while others have streamlined approval processes. Oregon, California, and several other states have statewide laws allowing ADUs on single-family lots, but city-level rules still apply.

Practical takeaway: Research whether your city allows ADUs and what types are permitted. Contact your local planning department or visit their website to review zoning ordinances specific to your neighborhood before investing time and money in planning.

Navigating Zoning Laws and Local Regulations

Zoning regulations determine where and how you can build an ADU. These rules vary dramatically between cities and even between neighborhoods within the same city. Some areas restrict ADUs to owner-occupied properties only, while others allow absentee landlords. Some limit ADU square footage to 50% of the primary home's size; others use fixed limits like 1,000 square feet. Parking requirements, lot size minimums, and setback distances (how far the structure must be from property lines) also vary.

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As of 2024, approximately 32 states have passed legislation allowing or encouraging ADUs on single-family residential lots. California led this trend in 2016, followed by Oregon, Minnesota, and others. However, even in these states, individual municipalities maintain the power to set additional restrictions. A property that permits ADUs in one neighborhood may prohibit them two blocks away due to different zoning designations.

Your first step involves obtaining your property's zoning designation and reviewing the specific code section that governs residential uses. Most cities post zoning ordinances online through their planning or community development departments. These documents typically explain:

• Whether ADUs are allowed in your zone
• Maximum square footage and height limits
• Lot size requirements
• Parking requirements per unit
• Owner-occupancy rules
• Setback and distance requirements
• Outdoor space standards

After reviewing written codes, schedule a meeting with your city's planning staff. Planning departments often provide informal feedback before you invest in professional designs. Many cities now have ADU liaisons or staff specifically trained to help homeowners navigate the process. These consultations are typically free and can clarify ambiguous rules or explain how staff interprets regulations in practice.

Some jurisdictions have adopted model ordinances or streamlined ADU approval processes. For example, Minneapolis eliminated single-family zoning entirely in 2019, allowing up to three units on properties previously zoned for one home. Los Angeles created a Ministerial ADU Approval process that bypasses certain environmental reviews, reducing approval timelines from 18 months to 60 days for projects meeting specific criteria.

Practical takeaway: Download your city's zoning code and locate the section governing ADUs or residential development. Make a list of specific requirements (square footage limits, parking, setbacks) and verify your property meets them before proceeding. Contact planning staff with questions—they can clarify whether your idea is permitted.

Evaluating Your Property and Site Conditions

Not all properties are suitable for ADUs. Beyond zoning compliance, your land must meet practical and technical requirements. Begin by assessing lot size. Most jurisdictions require minimum lot sizes ranging from 5,000 to 7,500 square feet, though some allow ADUs on smaller urban lots. Measure your property or obtain dimensions from your property deed and plat map, available through your county assessor's office.

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Next, evaluate site topography and existing structures. A relatively flat property with good drainage is ideal and costs less to develop. Properties with significant slopes, poor drainage, or soil stability issues require additional engineering and site preparation. If your lot contains wetlands, streams, or protected vegetation, ADU placement becomes constrained and permits may be delayed.

Determine what utilities already serve your property and where new connections must be made. ADUs require separate water, sewer, and electrical service in most jurisdictions. Properties with existing utilities nearby have lower connection costs—typically $2,000 to $8,000 for water and sewer connections per unit. Properties requiring long runs or connections to distant main lines may face $15,000 to $30,000 in utility costs alone. Contact your water, sewer, and electrical providers to obtain:

• Service availability and connection locations
• Estimated connection costs
• Required capacity upgrades
• Timeline for service installation

Evaluate existing structures on your property. Garage conversions and basement ADUs cost 30% to 50% less than new construction because they reuse existing foundations and walls. However, conversion projects require careful assessment of structural integrity, electrical systems, plumbing, HVAC capacity, and moisture issues. An engineer or contractor should inspect the existing structure to identify necessary upgrades.

Site access and parking affect both construction cost and livability. Ensure adequate access for construction vehicles and permanent resident parking. Many jurisdictions require 1 to 2 parking spaces per ADU, though some cities in transit-rich areas waive this requirement. Calculate available parking on your property, accounting for spaces needed by the primary home's occupants.

Underground obstacles affect construction cost. Before final planning, schedule a utility locating service to mark the location of buried water lines, sewer lines, electrical conduits, and gas lines. This service is often free and identifies potential conflicts that could increase costs or require design modifications.

Practical takeaway: Measure your lot, walk the property to assess terrain and existing structures, and contact utilities about service availability and connection costs. These assessments cost under $500 but reveal whether ADU development is practical before investing in design and engineering.

Assembling Your Design and Permitting Team

ADU projects require input from multiple professionals, though the scope varies by project complexity. A simple garage conversion might need only a designer and contractor, while a new detached unit typically requires an architect, civil engineer, and structural engineer. Understanding who does what helps you build an appropriate team and budget correctly.

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Architects design buildings, typically holding state licenses requiring seven years of education and experience. They create detailed plans showing how the building will look and function. For simple ADU projects under 1,000 square feet in straightforward configurations, some jurisdictions allow designers (unlicensed professionals) to prepare plans. However, complex projects, new construction, or structural modifications typically require licensed architects. Architect fees range from $3,000 to $8,000 for straightforward ADU designs.

Civil engineers handle site planning, drainage, utility connections, and grading. They design the layout of how the ADU sits on the property, where utilities connect, and how stormwater drains. Civil engineering services cost $2,000 to $5,000 for typical ADU projects. Structural engineers design the building's frame and foundation, ensuring it safely supports loads. Simple projects may not need separate structural analysis if the designer performs it, but most new construction requires structural engineering costing $1,500 to $4,000.