Building an Accessory Dwelling Unit (ADU) in Portland
Garage Conversions, New Builds, Zoning Rules & Complete Cost Guide
Accessory Dwelling Units (ADUs) have become very popular with homeowners in Portland, Oregon. Many people want to improve their homes and increase property value. Others want to earn rental income or provide space for family members, like aging parents.
If you want to convert your garage or build a new unit, you need to know about ADU development. This includes definitions, zoning rules, permits, costs, and design factors. This guide covers these parts. It gives you the knowledge to make smart choices about building an ADU in Portland.
What Is an Accessory Dwelling Unit?
An Accessory Dwelling Unit (ADU) is a self-contained residential unit located on the same lot as a primary single-family home, acting as secondary dwelling units. These units include essential living facilities such as a kitchen, bathroom, and sleeping area, enabling independent living. You create ADUs through various methods:
Detached New Construction: Standalone structures separate from the main house
Garage Conversions: Transforming an existing garage into a livable space
Basement Conversions: Renovating a basement with a separate entrance into an independent unit.
Additions to Existing Structures: Building onto the primary residence to create a separate unit.
These units are often referred to as granny flats, in-law suites, or secondary suites. They serve multiple purposes, including housing family members, providing rental income, and increasing the overall value of the property.
Zoning Regulations for ADUs in Portland
Portland's zoning regulations are designed to facilitate the development of ADUs across various residential zones. However, the specific allowances and requirements can vary based on the zoning designation of your property.
Understanding Zoning Designations
Portland's zoning designations dictate the types of structures and uses permitted on a property. The primary residential zones include:
Single-Dwelling Zones: These zones are intended for single-family residences and include:
RF (Residential Farm/Forest): Allows for larger lot sizes, typically not conducive to ADU development due to density restrictions.
R20, R10, R7, R5, R2.5: These zones allow for one primary dwelling per lot, with varying minimum lot sizes. These zones are subject to specific development standards.
Multi-Dwelling Zones: Designed to accommodate multiple residential units, these zones include R3, R2, R1, and RH. ADUs are permitted in these zones and can provide additional housing options within multi-family developments.
Commercial and Employment Zones: Certain commercial (e.g., CM1, CM2) and employment zones (e.g., EX) allow for residential uses, including ADUs, particularly when integrated with mixed-use developments.
ADU Allowances by Zone
The table below outlines the general allowances for ADU construction in various zones:
| Zone | ADU Allowance | Notes |
|---|---|---|
| RF | Limited, subject to larger lot size requirements | ADUs may be less feasible due to density restrictions. |
| R20, R10, R7, R5, R2.5 | Generally permitted | Must comply with specific development standards, including size and placement restrictions. |
| Multi-Dwelling Zones | Permitted | ADUs can be part of larger multi-family developments. |
| Commercial Zones (CM1, CM2) | Permitted when part of mixed-use developments | Integration with commercial uses may be required. |
| Employment Zone (EX) | Permitted | Often allows for residential uses, including ADUs, particularly in mixed-use contexts. |
Consult the Portland Zoning Code and confirm your property's zoning to determine the requirements for ADU development.
Determining Your Property's Zoning
To identify your property's zoning designation:
Access Portland Maps: Visit Portland Maps and enter your property address.
Check Zoning Information: Visit the "Permits & Zoning" section. Here, you can find detailed zoning information. This includes the base zone, overlay zones, and plan districts.
Permit Requirements for ADU Construction
ADU remodeling or building ADUs in Portland requires obtaining several permits to ensure compliance with local building codes and regulations. The primary permits include:
Building Permit: Mandatory for all ADU projects, covering structural aspects of the construction.
Electrical Permit: Required for any new or modified electrical systems within the ADU.
Plumbing Permit: Necessary for installing or altering plumbing systems, including water and sewer connections.
Mechanical Permit: HVAC installations or changes are needed to ensure proper ventilation and climate control.
If your property is in a historic or conservation district, you may need design review approval. It's advisable to consult with a Portland ADU contractor and the Portland Permitting & Development to understand all applicable requirements.
Typical Layouts and Design Plan Ideas for ADUs
These layouts and square feet recommendations can help you plan your garage conversion or new build ADU well.
Typical ADU Sizes and Layouts:
| ADU Type | Average Size (SqFt) | Typical Layout |
|---|---|---|
| Studio ADU | 250–400 sqft | Open floor plan with combined bedroom/living/kitchen area and separate bathroom. |
| One-Bedroom ADU | 400–600 sqft | Separate bedroom, open living/kitchen area, one bathroom. |
| Two-Bedroom ADU | 600–800 sqft | Two separate bedrooms, open living/kitchen space, and at least one bathroom. |
Popular ADU Design Considerations:
Energy Efficiency: Incorporate energy-efficient appliances, high-quality insulation, and HVAC systems.
Privacy & Separation: Use thoughtful landscape design or strategic entry placements to maximize privacy.
Multifunctional Spaces: Designs often include spaces that can flexibly serve as guest rooms, offices, or rental units.
Find a local ADU builder to help you with accessory dwelling unit plans. They will ensure your plans meet Portland’s building codes and zoning rules.
Is a Basement with a Separate Entrance Considered an ADU?
Yes, a basement with a separate entrance can be classified as an ADU if it meets specific criteria:
It has independent living facilities (kitchen, bathroom, sleeping area).
It complies with Portland’s ADU building codes and zoning regulations.
The city officially permits it as an accessory dwelling unit.
Converting a basement to an ADU and having the interior finished typically falls within the "attached ADU" category instead of the “detached ADUs” category, requiring permits similar to those of other ADU types.
Steps to Get Started Building Your ADU in Portland
1. Verify Your Property Zoning
Visit PortlandMaps.com and enter your address.
Check your zoning type and any overlay restrictions.
2. Consult with an ADU Specialist or Contractor
Search for "ADU builder near me" or trusted local experts.
Discuss design, timelines, and specific zoning details.
3. Obtain Necessary Permits
Key permits include:
Building Permit
Plumbing Permit
Electrical Permit
Mechanical Permit
Possible Historic District Approval (if applicable)
4. Finalize Plans and Budget
Decide between a garage conversion or a new build based on your budget, space constraints, and long-term goals.
Engage architects or designers for detailed accessory dwelling unit plans.
Additional Considerations & Tips
Parking and Access: Consider driveway extensions or parking availability for ADU occupants.
Utilities and Connections: Plan early for utility connections, as new builds might require extensive trenching and setup.
Rental Income Potential: Understand rental market values in Portland to set realistic income expectations if renting the unit.
Enhancing Your Portland Property with an ADU
Accessory dwelling units provide versatile solutions for adding a guest house and increasing your existing home's value, flexibility, and living spaces. Whether you opt for a garage conversion or a completely new build, understanding the zoning nuances, permit processes, typical costs, and design opportunities will empower you to make confident decisions for this new living area.
