Accessory Dwelling Units (ADUs) are one of Portland’s most popular home improvements—offering flexibility, rental income, and multigenerational housing solutions. But homeowners often face a key question: should you build a basement ADU or invest in a detached unit?
Let’s break down the pros, cons, and key considerations for each to help you decide which type of ADU is best for your property.
What Is a Basement ADU?
A basement ADU is a fully functional apartment built into the lower level of your existing home. It includes its own entrance, bathroom, kitchen, and living area—and is separated from the main home through a private stairwell or exterior access point.
What Is a Detached ADU?
A detached ADU (also known as a backyard cottage or guest house) is a standalone structure typically built in the backyard. It has complete independence from the main house, both structurally and in terms of utilities.
Basement ADU: Pros and Cons
- ✅ Lower Construction Costs: No need for a new foundation or roofing system
- ✅ Quicker Permitting: Often less complex than detached builds
- ✅ Great for Long-Term Guests or Family: Offers privacy with proximity
- ⚠️ May Have Limited Natural Light: Requires smart lighting and window placement
- ⚠️ Ceiling Height Requirements: Portland code requires at least 6’8” finished height
Detached ADU: Pros and Cons
- ✅ Total Privacy: Ideal for rental income or guest stays without noise overlap
- ✅ Design Freedom: Customize floorplans and architecture from scratch
- ⚠️ Higher Costs: Requires new foundation, utilities, and exterior finishes
- ⚠️ May Trigger Additional Zoning Review: Especially in Portland’s overlay zones
Which Is Better for Your Portland Property?
If you have an underused or unfinished basement, a basement ADU is usually the more cost-effective and practical choice—especially if your home already has exterior access or plumbing rough-ins. For homeowners prioritizing privacy and rental appeal, a detached ADU offers unbeatable separation.
At Boss PDX, we help homeowners evaluate their space, budget, and goals to build the ADU that fits best. Check out our basement remodeling services to see how we transform Portland basements into livable, code-compliant ADUs.
Request a free estimate today to explore the potential of your basement or backyard space.
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