How to Build an ADU: A Planning Guide

Understanding ADUs: What They Are and Why They Matter

An Accessory Dwelling Unit (ADU) is a secondary residential structure on a single-family residential lot. ADUs go by many names—granny flats, in-law units, secondary units, or backyard cottages—but they all serve the same basic purpose: providing additional living space on an existing property. Unlike a traditional addition to a home, an ADU is a separate, self-contained unit with its own entrance, kitchen, bathroom, and living space.

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ADUs have grown significantly in popularity over the past decade. According to data from the Joint Center for Housing Studies at Harvard University, ADU construction has increased by more than 30% in states that have reformed zoning laws to permit them. California, which legalized ADUs statewide in 2016 through Assembly Bill 68, has seen tens of thousands of ADUs created, with estimates suggesting over 75,000 ADUs permitted since legalization began.

Property owners build ADUs for various reasons. Some want to create rental income from an underutilized backyard. Others build them to house adult children, aging parents, or other family members without selling their primary home. Many see ADUs as a way to increase property value while contributing to local housing supply. In cities facing housing shortages, ADUs represent one of the fastest-growing ways to add housing units without demolishing existing homes.

ADUs come in several types. Detached ADUs are separate structures, like a cottage or small building sitting behind or beside the main house. Attached ADUs share a wall with the primary residence, such as a converted garage or a side addition. Internal ADUs occupy space within the existing house itself—for example, a basement conversion or an upstairs unit in a split-level home. Junior ADUs (JUADUs) are smaller units, typically 500 square feet or less, that fit within existing structures with minimal external modifications.

Practical Takeaway: Before beginning any planning, identify why you want to build an ADU. Your motivation—rental income, family housing, property value increase, or housing shortage contribution—will shape every decision you make about design, financing, and construction timeline.

Navigating Local Zoning Laws and Regulations

Zoning laws are the most critical hurdle in ADU development. These are local regulations that dictate what you can and cannot build on your property. Zoning rules vary dramatically from city to city, county to county, and state to state. While some jurisdictions have embraced ADU-friendly policies, others maintain strict single-family zoning that prohibits secondary units entirely.

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To understand your local rules, you must contact your city or county planning department. This is not optional—building without compliance can result in fines, forced removal of the structure, or legal action. Most planning departments maintain zoning ordinances online or in physical offices. Many now offer free preliminary consultations where staff can explain what's allowed on your specific property. During this consultation, planners review your property deed, existing structures, lot size, and setback requirements (the minimum distance between buildings and property lines).

Key zoning questions to research include: Are ADUs permitted in your zone? Are there restrictions on unit size, number of units, or lot size requirements? Must the property owner live on-site (owner-occupancy requirements)? Are there parking requirements? What setback and height restrictions apply? Can you have both a detached ADU and an internal ADU, or only one? Are there design guidelines or architectural review requirements?

Recent state-level changes have shifted the zoning landscape. California's 2016 ADU legalization and subsequent 2019 Junior ADU law made it significantly easier to build ADUs statewide, though local jurisdictions can still impose restrictions. Oregon legalized ADUs in 2018, allowing up to two units per lot in many areas. Minnesota, Washington, and other states have passed ADU-friendly legislation. However, many conservative-zoned communities still prohibit ADUs or impose severe restrictions like requiring owner occupancy, limiting unit size to 500 square feet, or mandating that ADUs rent for at least $1,500 monthly.

Understanding setback and height restrictions is crucial. For example, a typical lot might require a 25-foot front setback, 5-foot side setbacks, and a 10-foot rear setback. Your ADU must fit within these boundaries. Height restrictions often cap buildings at 25-35 feet, though some jurisdictions allow variations based on structure type. If your lot is only 30 feet wide and 80 feet deep with standard setbacks, your detached ADU footprint may be severely limited.

Practical Takeaway: Schedule a meeting with your local planning department before investing money in architects or designers. Bring your property deed and ask for a written summary of ADU regulations, setback requirements, and any recent zoning changes. Many departments provide this information for free and can identify potential obstacles before you proceed.

Site Assessment and Property Evaluation

Once you've confirmed that ADUs are permitted in your zone, evaluate whether your specific property is suitable for ADU development. Many properties that legally permit ADUs are physically challenging to develop due to lot size, shape, terrain, utilities, or existing structures.

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Lot size and shape are primary considerations. Most jurisdictions with detached ADU provisions require lots of at least 5,000-7,500 square feet, though some require 10,000 square feet or larger. Your lot's configuration matters too. A long, narrow lot may allow less usable building area than a square lot of the same size. Measure your property and map out setback boundaries to determine how much land remains available for an ADU. For attached or internal ADUs, lot size is less critical, but you need sufficient space within your existing structure or alongside your primary residence.

Topography affects both feasibility and cost. Flat or gently sloping lots are easier and less expensive to develop. Steep slopes complicate foundation work, drainage, and landscaping, potentially increasing construction costs by 20-40%. Soil conditions matter as well. If your area requires soil testing due to flood risk, poor drainage, or contamination history, testing costs typically run $1,000-$3,000 but are essential before proceeding.

Utility access is critical. Your ADU will need water, sewer, electrical, and potentially gas connections. Properties with existing connections near the ADU location are far simpler to develop. If connections require running lines across your lot or connecting to distant main lines, costs rise significantly. A sewer connection can cost $5,000-$20,000 depending on distance and conditions. Electrical upgrades to your main panel might cost $2,000-$5,000. Call your local utility companies and request a site assessment. Many provide these assessments at no charge.

Check for easements and encumbrances on your property deed. Easements allow others (like utility companies or neighbors) to use portions of your land for specific purposes. A utility easement running across your potential ADU location could prohibit building there. Similarly, restrictive covenants or homeowners association rules might limit ADU development regardless of local zoning. Review your deed carefully or hire a title company to summarize restrictions—typically costing $200-$400.

Existing trees, drainage patterns, and neighboring structures influence design. Large, mature trees may be worth preserving for aesthetics and property value. Check whether your municipality has tree protection ordinances. Natural drainage patterns should inform where you place structures to avoid flooding issues. The proximity and orientation of neighboring homes affect privacy, views, sun exposure, and parking considerations.

Practical Takeaway: Hire a surveyor ($300-$800) to mark your property boundaries and verify setback lines. Combine this with a written summary from your planning department and utility assessments from local providers. This modest investment reveals whether your property is actually suited for ADU development before you spend thousands on architectural design.

Design, Permits, and Professional Requirements

Once you've confirmed your property permits ADUs and is physically suitable, you'll need professional design and permitting. This stage transforms your concept into construction-ready plans that meet building codes, zoning requirements, and safety standards.

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Most jurisdictions require designs to be prepared by licensed professionals—typically architects or, in some cases, experienced residential designers. Your planning department's website should specify requirements. Some areas allow homeowners to design and permit smaller ADUs (like junior ADUs under 500 square feet) independently, though this is increasingly uncommon. An architect typically costs $3,000