A basement apartment can house a parent, an adult child, a long-term guest, or a renter, all without adding a square foot to your home's footprint. In Portland, a finished basement with its own kitchen, bath, and sleeping area can qualify as an accessory dwelling unit (ADU), which means the project is governed by both building code and ADU rules. This guide covers what a basement apartment needs, the Portland rules to know, what it costs to plan for, and how Lamont Bros. approaches basement conversions.
People use the two terms interchangeably, but the city does not. A basement apartment is a general term for a self-contained living space below grade. A basement ADU is a legally permitted second dwelling unit on your lot, with its own kitchen, bath, and sleeping area. Only a permitted unit counts for the rules below, so if you plan to rent the space or house family long term, plan for the permitted route. Lamont Bros. compares the main ADU formats in the common types of ADUs and what they cost.
Every sleeping room in a basement needs an emergency escape and rescue opening, and an opening with a sill below grade needs a window well. Check the City's guidance on emergency escape and rescue openings before you plan the bedroom, because the size and location of the opening can shape the whole layout.
Basements built decades ago often have low ceilings, ducts, and beams that reduce usable height. Habitable rooms generally need a minimum ceiling height under the residential code, and lowering the slab or reworking mechanical runs is sometimes necessary. Confirm the current requirement with the City of Portland early, since it can decide whether a basement is a good candidate at all.
Considering a basement apartment? Start with our free Basement Remodel Buyer's Guide. It covers scope, budget, and design decisions before you meet with a design consultant.
A private entrance, ideally with its own path and landing, makes the space work for a tenant, a caregiver, or a family member who wants independence. Plan the entry together with the stairs, the window wells, and drainage so the exterior work does not create a moisture problem.
The plumbing layout is where cost and complexity concentrate. Placing the kitchen and bath near existing drain stacks keeps the slab work smaller. If the floor sits below the sewer line, an upflush or ejector system moves waste up to the main drain. Lamont Bros. explains these options in basement bathroom ideas, layouts, and plumbing options.
A basement that is a storage space can tolerate dampness. A basement someone lives in cannot. Address water entry first, then add insulation, heating, ventilation, and sound separation between the apartment and the floor above. See how to waterproof your basement for the steps.
Portland allows one ADU on a site with a house. An ADU can be up to 75 percent of the main home's living area or 800 square feet, whichever is smaller, and the 800-square-foot cap does not apply to a basement ADU in a primary structure at least five years old. Homeowners who use the city's system development charge (SDC) waiver must agree not to use the ADU or house as a short-term rental for 10 years. Rules change, so confirm them on the City of Portland's ADU page before you design. When a bathroom is added to a basement, the City's April 2025 building official determination on adding a bathroom also calls for a code-compliant path to the stairway or an exterior door.
Lamont Bros. ADUs generally start around $150,000, and permitting alone has run over $30,000 for some Portland ADUs. Where your basement lands depends on ceiling height, slab work, the entrance, and how much plumbing and electrical is new. Nationally, the 2025 Cost vs. Value Report found an ADU averaged about $166,406 and recouped 41 percent at resale. That is a national figure, not Lamont Bros. pricing. Use the ADU cost calculator for a starting estimate.
Lamont Bros. is a design-build remodeling firm in West Linn, serving the Portland metro and Southwest Washington. The process starts with a consultation and a home visit, moves into a design phase with 3D renderings and floor plans, and finishes with fixed-price construction for construction. Seeing the apartment's layout, egress, and plumbing plan on paper before construction begins is the best way to avoid surprises. For more ideas on how to use basement space, see finished basement ideas.
Lamont Bros. works with Portland-area homeowners who want a basement apartment that is designed, permitted, and built as a whole, with egress, plumbing, and moisture control planned from the first sketch. One cohesive team handles each project from start to finish.
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A basement can be rented as a legal unit when it is permitted as an ADU and meets building code. Homeowners who use the city's SDC waiver must agree to the 10-year restriction on short-term rentals.
Portland caps an ADU at 75 percent of the main home's living area or 800 square feet, whichever is smaller. The 800-square-foot cap does not apply to a basement ADU in a primary structure at least five years old.
Yes. An ADU is a self-contained dwelling unit, so it needs a kitchen, a bathroom, and a sleeping area.
Every sleeping room needs an emergency escape and rescue opening, and an opening with a sill below grade needs a window well.
Yes. Lamont Bros. designs and builds basement ADUs in the Portland area as part of its design-build process.