Attic Conversions near you

Before anyone talks about dormers or finishes, walk the attic with a tape and a flashlight and answer these. Any one of them can change the project from a renovation into a structural job.

This is the pair of rules that decides whether a big attic is a small room. Seattle's adopted residential code, reprinting the model text, requires habitable space and hallways to have a ceiling height of not less than 7 feet, with bathrooms, toilet rooms and laundry rooms permitted at 6 feet 8 inches. For rooms with sloped ceilings it sets a different test: the required floor area of the room must have a ceiling height of not less than 5 feet, and not less than 50 percent of the required floor area must have a ceiling height of not less than 7 feet.

Separately, the code says how area is counted. Portions of a room with a sloping ceiling measuring less than 5 feet, or a furred ceiling measuring less than 7 feet from the finished floor to the finished ceiling, do not count toward the minimum required habitable area for that room. Since habitable rooms must be at least 70 square feet and at least 7 feet in any horizontal dimension, the low triangles along the eaves become storage, built-in cupboards or knee-wall closets rather than part of the room on paper.

The other definition worth knowing is that a habitable attic is normally considered a story above grade plane, which affects how the whole building is classified. Seattle's code allows an exception where the habitable attic is not larger than half the floor area of the story below, the dwelling has a fire sprinkler system, the occupiable space is enclosed by the roof assembly above, knee walls at the sides and the floor-ceiling assembly below, and the attic floor does not extend beyond the exterior walls of the story below. Whether your jurisdiction has that exception is worth asking early, because it can change the approval path for the whole house.

This is the structural heart of an attic conversion and the reason most of them need an engineer. Seattle's adopted table of minimum uniformly distributed live loads gives uninhabitable attics without storage 10 pounds per square foot and uninhabitable attics with limited storage 20 pounds per square foot. Habitable attics and attics served with fixed stairs are given 30 pounds per square foot, the same figure the table gives for sleeping rooms, while rooms other than sleeping rooms are given 40.

The gap between 10 or 20 and 30 is what the work has to close. Depending on the house, that is done by sistering new members alongside the existing joists, by dropping a new floor structure between them, by adding a beam or a bearing line beneath, or by transferring load to walls that are already carrying something. Which of those is right depends on what is below, and the answer comes from a licensed design professional rather than from a rule of thumb.

The roof structure has a parallel problem. In a rafter roof, collar ties and ceiling joists often act together to stop the walls spreading under load, so removing them to open up headroom without replacing that restraint is one of the more serious mistakes available in residential construction. In a truss roof the webs are doing the same job more explicitly. Either way, the framing that is in the way is usually in the way because it is working.

A habitable attic is a top-floor room with one way out, so the code treats it the way it treats a basement. Seattle's adopted residential code requires basements, habitable attics and every sleeping room to have not less than one operable emergency escape and rescue opening, operable from inside the room without keys, tools or special knowledge, with a net clear opening of at least 5.7 square feet, at least 24 inches high and 20 inches wide. Where a window opening control device is fitted it must sit no more than 70 inches above the finished floor. On a steep roof this usually means a dormer, because a skylight in the plane of the roof rarely delivers that opening at a height a person can reach and use.

The stair is the other half. The code requires stairways to be at least 36 inches in clear width above the handrail height, with headroom of not less than 6 feet 8 inches measured vertically from the sloped line adjoining the tread nosings, riser height of not more than 7 and three-quarter inches, and tread depth of not less than 10 inches, with the greatest riser and greatest tread in a flight within three-eighths of an inch of the smallest. Those numbers determine the length of the stair, and the length of the stair determines which room downstairs gives up floor space. It is the single most common reason a promising attic conversion gets redesigned.

Before the conversion, the insulation sits on the attic floor and the attic above it is vented to the outdoors. Afterward the attic is inside the house, so the insulation has to move to the roof plane, and that is a genuine building science decision rather than a product choice.

If the roof stays vented, air has to keep moving from eave to ridge above the insulation. Seattle's adopted code requires enclosed attics and enclosed rafter spaces to have cross ventilation for each separate space, sets the minimum net free ventilating area at one one-hundred-fiftieth of the area of the vented space, and allows one three-hundredth where a Class I or II vapor retarder is installed on the warm-in-winter side in the colder climate zones and between 40 and 50 percent of the required ventilating area sits in the upper portion of the space, no more than 3 feet below the ridge. Critically, it requires not less than a 1-inch space between the insulation and the roof sheathing at eave and cornice vents, which is why baffles are installed at every rafter bay rather than at some of them.

The alternative is an unvented assembly, which the code permits under specific conditions: the unvented attic space is completely within the building thermal envelope, no interior Class I vapor retarder is installed on the attic-floor side, and the insulation meets one of several prescribed combinations. Where air-permeable insulation sits directly below the structural sheathing, a minimum of R-10 rigid board or sheet insulation is required directly above the sheathing for condensation control, and where both air-impermeable and air-permeable insulation are used, a minimum of R-10 of air-impermeable insulation goes in direct contact with the underside of the sheathing with the air-permeable insulation below it. Which route is right depends on your climate zone, the roof covering and whether the roof is being replaced at the same time.

An attic conversion needs a building permit, and in most jurisdictions it needs stamped structural drawings because the floor loads and often the roof framing are being changed. Expect separate electrical and mechanical permits, and expect the sequence of rough inspections that goes with new habitable space.

If the plan is a separate unit rather than extra rooms, state ADU law may help. California's statute allows an accessory dwelling unit attached to or located within the proposed or existing primary dwelling, including attached garages, storage areas or similar uses, and caps attached unit size at 50 percent of the existing primary dwelling. Washington requires cities inside urban growth areas to allow at least two accessory dwelling units on lots in zoning districts that allow single-family homes, and prohibits requiring the owner to live on the lot. An attic unit still has to meet the same egress, height and load requirements as any other habitable space, so the statutes change the zoning answer rather than the construction one.

The last permit-stage item is the one people forget: a dormer changes the exterior of the house, which can bring in setbacks, height limits, lot coverage in some places, and design review in a historic district. Confirm all of that before the framing plan is finalized.

The largest variables are structural and are settled before finishes are discussed: how much reinforcement the floor needs to carry 30 pounds per square foot, whether the roof framing has to be altered and how, how many dormers the escape opening and headroom require, where the stair lands and how much of the room below it consumes, and whether the roof assembly is being converted to unvented with the roof covering replaced at the same time. Heating and cooling for a new top floor is a real line item too, since the existing system was sized without it.

Post the project on LokalMatch with your ZIP code, the roof framing type if you know it, the ridge height you measured, and whether you want bedrooms, an office or a separate unit. Contractors who take on attic conversions in your area receive the request and get in touch. They pay for the leads; homeowners use LokalMatch at no cost. LokalMatch does not build anything, does not quote and does not screen, rank or recommend the businesses that contact you, so confirming licensing, insurance and the structural engineer's involvement is on you.