Click here to open a PDF version of this tip sheet.
Things to consider when converting an unfinished basement:
- All work must comply with the most recently adopted City construction codes. This tipsheet is specific to one and two family dwellings under the International Residential Code (IRC), Washington State Energy Code (WSEC), and other codes as applicable.
- Ceiling height minimum for habitable space in a existing basement being converted to habitable space must be 6’8” (80”) from finished floor to underside of ceiling joists, with no projections below 6’4” (76”). Projections should not be along hallways or center of habitable rooms.
- Inadequate ceiling height:If the basement does not meet the required ceiling height, and you intend to lower the basement floor to meet the required heights, then 7’ shall be required with this work. In addition, professional engineer design will be required to verify wall stability for unbalanced loads, excavation procedures and permanent bracing.
- Foundation walls require exterior wall drainage and exterior waterproofing to prevent moisture from migrating into the basement. Per IRC R405 and IRC R406. If proposing alternative methods, identify compliance by managing roof drainage, drainage around the foundation, and that the basement has no evidence of past moisture issues including sump pumps for ground water. This is normally performed by detailing on a site plan of downspout collection and discharging down slope and away from the foundation to vegetated yard space, verifying that wall section view shows grades sloping away with 6” fall in 10’ horizontal per R401.3, and no evidence of moisture on the basement walls or ground water sump pump. Additionally, there are systems that collect moisture from the inside face of the concrete walls. These systems need to be professionally designed by engineer licensed in the State of Washington. The installation must be sealed and tested to prevent moisture vapor with drainage to an approved location.
- Smoke Alarms – There must be at least one smoke alarm in the basement that is interconnected with the main floor alarm. If there are bedrooms, then provide an alarm in each bedroom and one outside the bedroom(s) in a common location.
- Carbon Monoxide Alarms – There must be at least one carbon monoxide alarm in the basement that is interconnected with the main floor alarm. It must be in the hallway/common area outside the bedroom(s), and in bedrooms with gas fixtures.
- Internal stairs are required from the main floor to the basement unless this is an accessory dwelling unit. If the stairs are existing, the risers cannot exceed 8” and the treads cannot be less than 9”. The tread depth and the riser heights must be consistent to within a 3/8” tolerance. The minimum headroom must be 6’8” measured from the nose of each tread. The width of the stairway may not be less than 32”. The stairs must have at least one handrail. Light switches and landings are required at the top and bottom of the stairs. New stairs must meet tread maximum rise of 7.75” and minimum run of 10” and other standards not identified here.
- Emergency Escape – The basement must have at least one emergency egress window or a door directly to the exterior. If you are creating a bedroom, the egress window or door must be within the bedroom itself. Egress windows shall provide 5.7 square feet of openable area, with a minimum openable width of 20” and a minimum openable height to 24”. The sill height may be a maximum of 44” above the finished floor. If the egress window sill height is below adjacent grade, a min 36” deep window well is required consisting of a width of at least the openable portion of the window, but not less than 36” wide. Drainage of the window well must be detailed.
- Gas appliances located in the basement must be shown to comply with requirements per IRC Fuel Gas Chapter 24 for location, combustion, ventilation, dilution air, and meet manufacturer’s installation requirements for clearance.
- Exhaust Fans must be identified on plans for bathrooms, toilet rooms, cooking facilities as applicable. Include locations and proper exhaust rates per IRC M1503 and associated Washington State Amendments.
- Energy Code requires newly heated spaced meeting fuel normalization and additional energy credit requirements that must be accurately reflect in plans, details, and any additional forms per WSEC R406.
- Insulation and Fenestration is required to meet WSEC R402. The plans must detail the heated and unheated spaces propoosed. A popular method for insulating basement areas is with R5 continuous extruded polystyrene insulation against the basement concrete with 2” x 4” studs at 16” on center with a top plate secured to the floor joists above, a pressure treated bottom plate fastened to the slab below, and stud cavities insulated to R13 batt insulation. If this method is not selected, another common method is with 2×4 stud wall with adequate space from concrete to allow for R21 insulation that is not in contact with concrete walls. In both cases, an R5 thermal break between the wall and slab is typical. Windows and doors to meet 0.30 U-value or less.
- Heating source for the habitable areas and all bedrooms must be clearly identified on the plans. Heating ducts must not project below 6’4” and not be located at a walking area or in-line with access hallways.
- Plumbing: Backwater valves are required if below the City’s maintenance connection or potential for backflow due to basement elevation. Identify if grinder pump is necessary and provide design information.
- Plans must show in detail that the work will comply with code. Plans must be indelible, reproducible, to scale, with two lines for walls, and 3 lines for windows, and doors locations clearly identified. Common plans include:
- Scope of work document describing all the proposed work with cost of materials and labor.
- Floor plans with plumbing and mechanical fixture locations and clearances, stairs and access points tied into the downstairs heated area, emergency escape, smoke and carbon monoxide alarms, heating, clearly identified heated and unheated areas.
- Section view of basement area with furred out walls, headroom, insulation details, window sill heights, floor joist framing size, spacing with spans and support.
- Site plan showing property line distances from structure with adequate emergency escape pathways to public way.
- Framing modifications require additional top view framing details for roof, floor and any other modifications. Engineered design is required if not meeting prescriptive requirements of the IRC.
- Completed WSEC compliance information.
Additional plan and submittal information is required to convert the basement to an Accessory Dwelling Unit (ADU).
