ADU Flooring in Bellingham: 2026 Costs, Sound, and Slab Prep

ADU flooring cost in Bellingham for 2026

ADU flooring in Bellingham costs $3.85 to $11.00 per square foot installed for the laminate and LVP that go into most units, which puts a 700 square foot unit at $2,700 to $7,700 for the main floor area. An accessory dwelling unit, or ADU, is a self-contained second home on the same lot as the main house, with its own kitchen and bathroom, and Bellingham has permitted them since 1995. The flooring budget for one is small next to the building itself, but it is the surface a tenant or a parent will live on every day, and in a converted garage or a daylight basement the substrate under it is where the money and the risk sit.

The per-square-foot rates below match the live LVP and laminate installation and carpet installation service pages, with the bath rates from the tile flooring page. Whole-house rates apply to an ADU with one adjustment: a unit under about 500 square feet often prices closer to the top of each range because the crew is mobilizing for a single short day, and the $350 to $500 minimum most Bellingham crews carry covers a small bathroom on its own.

Installed price per square foot by material

This table covers the finished floor only, installed over a sound, flat, dry substrate. Slab leveling, moisture mitigation, sound underlayment, and stairs are separate line items covered further down.

Flooring prices verified September 2026 against the live Bellingham Floor Pros service pages and active Whatcom County ADU and garage conversion bids.

Four Bellingham ADU budgets, worked out

These use the rates above and the substrate prices from the sections that follow. Every one of them moves with the condition of what is under the new floor.

• Two-car garage conversion, 440 square feet, 1960s Roosevelt or Columbia house: $220 slab moisture test, $726 to $2,178 in self-leveling underlayment over the pitched slab, $1,760 to $4,400 for 400 square feet of 20 mil SPC LVP, and $440 to $1,100 for a 40 square foot porcelain tile bath. $3,100 to $7,900, plus $880 to $2,750 if the slab fails its moisture test and needs mitigation before anything goes down.
• Daylight basement ADU, 700 square feet, Cornwall Park or Columbia: $220 for the moisture test, $2,816 to $7,040 for 640 square feet of LVP, and $660 to $1,650 for a 60 square foot tile bath. $3,700 to $8,900, with $880 to $2,750 in slab prep and vapor work on the invoice about as often as not in a house that age.
• New detached ADU, 800 square feet, on an alley lot in Fairhaven or South Hill: 700 square feet of LVP at $3,080 to $7,700 or laminate at $2,695 to $5,005, plus a 60 square foot tile bath at $660 to $1,650. $3,400 to $9,400 in vinyl or laminate, or $6,800 to $12,400 with engineered hardwood through the living space. A new slab poured with a vapor retarder under it skips the mitigation line entirely.
• Attached unit over the garage, 600 square feet, Sehome or Lettered Streets: 450 square feet of LVP at $1,980 to $4,950, a sound-rated underlayment under it at $383 to $990, and a 150 square foot carpeted bedroom at $578 to $1,320. $2,900 to $7,300, before the stairs, which run $275 to $550 per flight in carpet or $275 to $495 per tread in matching LVP.

The LVP installation cost guide breaks the $4.40 to $11.00 spread down by wear layer and core, and the guide to reading a flooring estimate explains which of these lines should appear on a quote as their own number.

What Bellingham's ADU rules mean for the floor

Flooring never needs its own permit, but the ADU does, and the rules that govern the unit decide which code the floor assembly is reviewed under and how big the job can get. The City's ADU page, updated September 17, 2026, is the source for everything in this section; the Permit Center published new ADU quick guides two days earlier.

Where ADUs are allowed and the 1,000 square foot cap

According to the City of Bellingham, ADUs are allowed in every area that permits residential development, with one exception: the Lake Whatcom watershed. That rules out Sudden Valley and the lakeside streets off Silver Beach, which matters because those neighborhoods hold a lot of the split-level daylight basements that make easy conversions elsewhere. A lot can carry up to two ADUs, each capped at 1,000 square feet, and a detached unit can have up to 800 square feet of garage or workshop attached to it. Owner occupancy is no longer required, and the unit can be sold as a condominium. Ferndale and Lynden write their own ADU codes, so a unit outside the city limits follows a different rulebook.

For the floor, the cap sets the ceiling on the job. The largest legal ADU in Bellingham is a 1,000 square foot install, which in LVP is $4,400 to $11,000 and takes a crew two to four days on a ready substrate. The typical unit is smaller than that: the City's own examples are a converted garage, a basement or attic unit, and an addition with its own entrance.

Permits: the floor rides on the building permit

An ADU needs a land use permit and a building permit, and the land use approval has to come first. The finished floor has no permit item of its own; the building inspector signs off on what is under it in a conversion: a slab that gets a new vapor retarder and topping, a subfloor that gets replaced, or a floor-ceiling assembly that gets insulation and a new ceiling below. The flooring permit guide covers where flooring-adjacent work crosses into permit territory on its own; inside an ADU project, all of it sits under the master permit and gets inspected on the building's schedule, which is one reason to have the flooring contractor lined up before the framing inspection rather than after the drywall.

Attached or detached: which code, and why the floor cares

The City notes that an ADU may be reviewed under the residential code or the commercial building code depending on its configuration. The difference matters for exactly one flooring question: sound. Under the commercial code, floor-ceiling assemblies between dwelling units need a sound transmission class (STC) of 50 and an impact insulation class (IIC) of 50 in the lab, or 45 when tested in the finished building. The residential code sets no sound rating between a main house and an attached unit. A detached unit has no shared assembly at all, so the sound section below applies only to attached units, and the Permit Center will tell you which code your drawings fall under before the flooring order goes in.

Garage conversions: the slab decides the floor

In older Bellingham neighborhoods a converted garage is the most common attached ADU and the one where the flooring budget is least predictable, because a garage slab was poured to park a car on, and the three things that made it a good garage floor make it a bad living room floor.

Slope and headroom

Garage slabs are pitched toward the door so water runs out, typically about 1/8 inch per foot. Across a 20 foot bay that is roughly 2.5 inches of fall from the back wall to the door, and no floating floor tolerates that. Self-leveling underlayment is a cement-based pour that seeks its own level over a primed slab, and it runs $1.65 to $4.95 per square foot installed in Bellingham, including the primer. It is the right fix for up to about an inch of correction. Past that, the material cost climbs fast, and the better answer is a sleeper floor: pressure-treated sleepers over a vapor retarder with rigid foam between them and a plywood deck on top, at $4.40 to $7.70 per square foot. The sleeper system also brings the new floor up toward the level of the house, which matters at the door between the two units, since a garage slab usually sits several inches below the house floor.

The catch is headroom. Habitable rooms need a 7 foot ceiling under the residential code, and a 2x4 sleeper deck takes about 4.25 inches off whatever the garage has. A garage with an 8 foot ceiling can afford it. One at 7 foot 4 cannot, and that unit gets a leveler and a step at the connecting door instead. The subfloor leveling guide covers how a crew maps the slab with a 10 foot straightedge before choosing between the two.

Moisture testing an old slab

Most Bellingham garages built before the 1990s were poured without a vapor retarder under the slab, over ground that stays wet from October through May. The slab wicks that moisture upward all winter, and a floor that seals the surface traps it. Every conversion in this climate should start with an in-situ relative humidity test, the ASTM F2170 probe method, at $220 for a typical garage. Laminate and glue-down vinyl specs generally cap the slab at 75 to 80 percent RH; many rigid-core floating LVP lines tolerate more, and the number the manufacturer prints in its install guide is the one the warranty rides on. A slab that reads above the spec gets a moisture mitigation coating, which runs $880 to $2,750 for a garage-sized area, before the leveler goes down on top of it. The subfloor moisture guide explains the test in detail, and the subfloor and moisture service page lists the mitigation options by price.

Oil is the other garage-specific problem. Adhesive and leveler will not bond to a stained slab, so the crew degreases and grinds the stains before any of it goes on, and a floating LVP floor over a foam pad is the one option that shrugs off a slab that cleaned up poorly.

A cold slab and what fixes it

An uninsulated garage slab in January sits close to the outdoor ground temperature, and a floor laid directly on it feels like it. Floating LVP over a foam underlayment with a built-in vapor barrier, at $0.35 to $0.95 per square foot for the pad, breaks the direct contact and is the budget answer. The sleeper floor with rigid foam between the sleepers is the real fix, and it is the same assembly that solved the slope problem above, which is why so many Bellingham garage conversions end up with one. Glue-down LVP transmits the slab temperature straight through and is the wrong pick on an uninsulated slab. The cold floors guide ranks the options by what they cost per degree gained.

Basement and attic units

A basement ADU has the same slab questions as a garage without the slope problem, and an attic or second-floor unit trades the moisture questions for weight and sound.

Daylight basements in Columbia and Cornwall Park

The mid-century daylight basements north of downtown convert well because the slab is already inside a heated envelope and the ceiling height is usually there. The moisture test still comes first; a basement slab in a house that age reads high often enough that the basement flooring guide treats slab prep as a line to expect rather than a surprise. Rigid-core LVP floating over a vapor-barrier pad is the standard floor, with sheet vinyl or tile in the bath, and the crew levels any section of slab more than 3/16 inch out over 10 feet at $1.65 to $3.30 per square foot. Laminate goes down here only over a separate 6 mil poly layer, and engineered hardwood only after the slab passes the test and gets a mitigation coat, at $9.90 to $15.40 per square foot for the wood alone.

Attic and second-floor units: weight and sound

An attic conversion in a Sehome or Lettered Streets house from the 1910s through the 1930s starts with an engineer's letter on whether the ceiling joists can carry a living load, because they were sized to hold up a ceiling, not a kitchen. Once the framing is sistered or replaced and a new subfloor is down, the floor choice is about pounds and decibels. LVP at roughly 1.5 to 2 pounds per square foot and carpet over pad are the light options; porcelain tile with its mortar bed runs 8 to 10 pounds per square foot and stays in the bathroom. Every attic and over-garage unit is an attached ADU with a floor over someone's ceiling, which is the sound problem the next section prices out.

Sound between the main house and an attached unit

Footsteps through the floor are the complaint that ends more ADU rental arrangements than any other, and the floor is the cheapest place to solve it because the ceiling below is usually already finished.

IIC and STC, in plain terms

IIC, the impact insulation class, measures how much footstep and dropped-object noise a floor-ceiling assembly blocks; STC, the sound transmission class, measures airborne sound like voices and television. Both are lab numbers where higher is better, and 50 is the commercial-code floor for both between dwelling units. A plain wood-joist floor with a drywall ceiling tests in the 30s for IIC. Carpet over a dense pad lifts the assembly into the 50s or better on its own, which is why carpet keeps winning ADU bedrooms over living space. Hard flooring gets there only with a rated underlayment under it and, in most assemblies, insulation in the joist bay and resilient channel under the drywall below.

Underlayment choices and what they cost

The builder-grade foam pad under a floating LVP floor carries no meaningful sound rating. A cork or rubber membrane that comes with a real IIC test report adds $0.85 to $2.20 per square foot over that pad, so the premium on a 600 square foot unit is $510 to $1,320. A 3 mm to 6 mm cork roll, at $0.55 to $1.65 per square foot, is the middle option. Two details decide whether the money works: the underlayment's test report has to be for an assembly like yours, joists and drywall rather than concrete, and the perimeter has to be isolated with foam tape so the floor does not couple to the walls. The HOA flooring rules guide walks through reading an IIC report, and the underlayment guide compares the products by climate as well as sound. Under the residential code none of this is required. It is still the line most owners of an attached unit wish they had not cut.

Picking the floor room by room

An ADU is one open room with a bath and usually a bedroom off it, so the material decision is smaller than in a whole house. The pattern that holds up across Bellingham units is one waterproof floor through the living area and kitchenette, with tile or sheet vinyl in the bath. Carpet goes in the bedroom only when the unit sits over living space.

Living area and kitchenette

Rigid-core LVP with a 20 mil wear layer is the default here for the same reasons it dominates Bellingham rentals: it handles a kitchenette spill and a tenant's furniture, and a damaged plank can be swapped. The LVP buying guide explains why the wear layer matters more than the thickness on the box. Laminate at $3.85 to $7.15 saves $500 to $1,500 on a typical unit and holds up in a dry, new-build detached unit; on any slab or in a unit with a kitchenette, the swelling risk at the seams makes the LVP premium worth paying. Engineered hardwood is the floor to choose when the unit is being built to sell as a condominium, where the finish shows in the listing photos.

Bathroom

The ADU bath is small, 40 to 60 square feet, and it prices at the crew minimum in sheet vinyl or LVP and at $440 to $1,650 in porcelain tile. Tile is the pick where the unit is meant to rent for years; sheet vinyl is the pick where the budget is tight and the unit is for a family member. Either way, the flange height and the transition to the living area floor get decided before the floor order, and the bathroom floor replacement guide covers both. An electric heated mat under the tile adds $4.40 to $11.00 per square foot and is the cheapest comfort upgrade in a unit on a slab.

Bedroom

Carpet over a dense pad at $3.85 to $8.80 is the warmest and quietest floor available and costs about the same as LVP. In a unit over the garage or over the main house, it goes in the bedroom by default. In a ground-floor unit with its own slab, running the LVP straight through is the simpler choice and the one that survives tenant turnover with less work.

Entry and stairs

The entry takes the wet months head on. A 30 to 40 square foot tile landing inside the door at the tile rate, or the LVP run straight to the threshold with a boot tray, both work; a carpeted entry does not last a Bellingham winter. Stairs to an over-garage unit are their own line: $275 to $550 per flight in carpet, which also quiets the stair, or $275 to $495 per tread in matching LVP treads with stair nosings, which looks better and costs several times more across a 13 tread run.

ADUs as rentals: durability and turnover

Most Bellingham ADUs are built to rent, and a rental floor is judged by how it looks after the third tenant, not the first.

Rental registration and the inspection

According to the City, every rented ADU inside the city limits has to register with the Rental Registration and Safety Inspection Program. An attached unit is exempt from the fee and inspection when the owner lives on the property and there are no other rentals on it; a detached unit is inspected regardless. The inspection is a safety checklist rather than a finish review, but a loose transition strip or a lifted plank at a doorway is a trip hazard, and a floor that was installed with the correct expansion gap and pinned transitions does not produce those.

What holds up between tenants

The rental property flooring guide makes the full case, and the part that matters for an ADU is that 20 mil rigid-core LVP is the only floor in the table above that a landlord can have repaired plank by plank, at $220 to $440 per plank including disassembly, instead of replaced room by room. Carpet in a rental bedroom gets replaced roughly every second tenancy. Laminate that took a dishwasher leak gets replaced, full stop. For an ADU, the extra $1,000 to $2,000 that LVP costs over laminate on a 700 square foot unit is one avoided replacement.

Any contractor working on the unit should be checked against Washington L&I for an active registration, bond, and insurance before the deposit goes out; the contractor verification guide shows how to do it in five minutes. On an ADU project the flooring sub is usually working under a general contractor's permit, and the general should be able to name who is installing the floor and show that registration on request.

ADU flooring FAQs from Bellingham homeowners

How much does it cost to floor an ADU in Bellingham?

In 2026, a 700 square foot ADU runs $2,700 to $7,700 for the main living area in laminate or rigid-core LVP, plus $440 to $1,650 for a tile bath. A garage conversion adds $220 for a slab moisture test and $700 to $2,200 to level the pitched slab, and an attached unit over living space adds $510 to $1,320 for a sound-rated underlayment. The largest legal ADU, at the City's 1,000 square foot cap, runs $4,400 to $11,000 in LVP.

What is the best flooring for a garage conversion ADU?

Rigid-core LVP with a 20 mil wear layer, floating over a foam pad with a built-in vapor barrier, on a slab that has been moisture tested and leveled. It tolerates the residual moisture in an old garage slab better than laminate, does not depend on a bond to a slab that may have oil in it, and can be repaired plank by plank. Where the slab slopes more than about an inch, a sleeper floor with rigid foam goes under it, which also warms the floor and can bring it level with the house.

Do I need soundproofing under the floor of an attached ADU?

Only if the unit is reviewed under the commercial building code, which requires STC 50 and IIC 50 between dwelling units; the residential code sets no sound rating. Either way, an attached unit over living space benefits from a rated cork or rubber underlayment at $0.85 to $2.20 per square foot over a standard pad, and carpet over dense pad in the bedroom solves most of the footstep problem on its own.

Can I put an ADU in Sudden Valley?

No. The City of Bellingham allows ADUs in every residential area except the Lake Whatcom watershed, which includes Sudden Valley and the lakeside portion of Silver Beach. Everywhere else in the city, a lot can hold up to two ADUs of up to 1,000 square feet each, with a land use permit and a building permit. Ferndale and Lynden set their own ADU rules.

Send the unit's square footage, whether it is a garage, basement, attic, or new build, and a photo of the existing slab or subfloor, and request a free estimate. The estimator meters the slab on the first visit and writes the leveling, moisture, and sound lines into the quote as their own numbers, so the flooring cost is settled before the drywall goes up.