ADU Builders Seattle | Cost, Permits & Timeline 2026 | Love Construction

# ADU Builders Seattle | Cost, Permits & Timeline 2026

ADU construction in Seattle costs $150 to $350 per square foot for many attached ADUs, basement conversions, and garage conversions in 2026. Detached backyard cottages and DADUs often cost more because they need a separate foundation, roof, utilities, drainage, and full exterior envelope. A practical timeline is 6 to 12 months from early planning through final inspection, depending on design, permit review, site conditions, and how much work the existing home or lot needs.

For a fast planning answer: start with the ADU type, then price the utility work, permit path, site access, and finish level. A simple interior conversion is usually the lowest-cost path. A detached rental-ready DADU is usually the highest-cost path because it behaves more like a small new home.

For homeowners comparing an attached ADU, basement conversion, garage conversion, or detached DADU, the right question is not only "how much per square foot?" The better question is: what type of living space are you creating, how much independent utility work is required, and how hard will it be to build on the site?

Love Construction helps Seattle and King County homeowners plan ADUs, DADUs, home additions, remodels, and high-performance construction. This guide explains the budget, permit, and timeline issues to understand before choosing an ADU builder.

What Counts as an ADU in Seattle?

An ADU is a secondary living unit on the same property as a primary home. It can be attached to the house, built inside existing space, or constructed as a separate backyard cottage.

An attached ADU often uses part of the existing home, such as a basement, lower level, garage, or addition. It may share more structure with the main house, but it still needs a code-compliant kitchen, bathroom, sleeping area, safe entry, heating, ventilation, electrical, plumbing, and fire separation where required.

A detached ADU, often called a DADU or backyard cottage, is a separate structure. These projects usually cost more because they need their own foundation, roof, envelope, utility connections, drainage plan, and construction staging.

A garage conversion can look simpler at first, but older garages often need slab work, insulation, moisture control, electrical upgrades, new openings, windows, and structural repairs before they become comfortable living space.

Seattle ADU Cost Ranges for 2026

For early planning, many Seattle ADU projects fall into these broad ranges:

• • Basement or interior conversion: often $150-$250 per square foot when the shell is usable and utilities are nearby
• • Attached ADU or garage conversion: often $200-$325 per square foot when structural, insulation, plumbing, and access upgrades are needed
• • Detached ADU or backyard cottage: often $275-$450+ per square foot when new foundation, utility runs, drainage, and full exterior construction are included

Those ranges are planning numbers, not bids. A compact unit with a simple bath and kitchenette can cost less than a detached cottage with a full kitchen, vaulted roofline, custom windows, and difficult site access.

The total budget usually includes design, engineering, permits, demolition, excavation, foundation, framing, windows, roofing, siding, insulation, electrical, plumbing, HVAC, drywall, flooring, cabinetry, counters, tile, fixtures, paint, appliances, cleanup, and final inspections.

What Drives ADU Costs Up?

The biggest cost driver is whether the project can reuse existing structure and utilities. A basement ADU with good ceiling height and nearby plumbing has a different budget than a detached backyard cottage that needs trenching, sewer connection, new electrical capacity, and stormwater planning.

Utility work is often the surprise. Sewer depth, water supply, electrical panel capacity, heat pump placement, ventilation, and drainage can change the budget quickly.

Site access matters in Seattle neighborhoods. Tight side yards, slopes, trees, retaining walls, limited staging, and older homes can add labor and equipment time.

Kitchen and bathroom scope also matters. Cabinetry, tile, waterproofing, ventilation, plumbing fixtures, counters, appliances, lighting, and electrical all make a small ADU feel complete, but they add more cost than plain square footage suggests.

Energy performance should be planned early. Better insulation, air sealing, high-performance windows, and efficient heating can make a small unit quieter and more comfortable. These details can be especially valuable when the ADU is intended for family or long-term tenants.

Seattle ADU Permits and Planning Steps

Seattle allows ADUs and detached ADUs on many residential lots, but the project still needs to satisfy zoning, building, utility, life-safety, and site requirements. Before design goes too far, review the property constraints.

Important questions include:

• • Is the unit attached, detached, or inside existing space?
• • Does the lot have enough room after setbacks, lot coverage, tree, and access limits?
• • Can sewer, water, electrical, and heating systems support the new unit?
• • Will the project need structural engineering?
• • Is stormwater or drainage work required?
• • Can materials and crews access the work area without damaging the property?

Permit timing varies. A straightforward conversion can move faster than a detached cottage with engineering, utility work, and site constraints. Many homeowners should plan for several months of design, documentation, and review before construction starts.

How Long Does an ADU Take?

A realistic ADU timeline is often 6 to 12 months from early planning through final inspection. Some simple conversions move faster. Detached ADUs, second-story additions, older-home tie-ins, and projects with utility or site complexity can take longer.

Most ADU projects move through these phases:

1. Feasibility and budget planning 2. Design and layout decisions 3. Engineering and permit documents 4. Permit review and corrections 5. Ordering and pre-construction planning 6. Site work, foundation, or demolition 7. Framing, rough plumbing, electrical, and HVAC 8. Insulation, drywall, finishes, fixtures, and final inspection

The schedule is strongest when finish selections happen before construction, not during construction. Cabinets, windows, doors, tile, plumbing fixtures, lighting, and appliances can all affect lead times.

Choosing an ADU Builder in Seattle

The best ADU builder is not just the lowest square-foot price. ADUs are small, dense projects with many trades working in tight spaces. The builder needs to understand structure, utilities, inspections, energy performance, and how the new unit will feel for real daily use.

Ask a builder how they handle utility planning, permit corrections, site access, weather protection, insulation, ventilation, and finish selections. Ask whether they have built attached and detached units, and whether they can explain where the budget is most likely to move.

For many homeowners, the cleanest path is a builder who can compare ADU, DADU, addition, and remodeling options before committing to one scope. Sometimes a detached ADU is the best answer. Sometimes a basement conversion, garage conversion, or primary-suite addition solves the same problem with less disruption.

When an ADU Makes Sense

An ADU can make sense when the household needs flexible living space, long-term family housing, rental potential, a private office, or a way to stay in a Seattle neighborhood without buying a larger home.

It is strongest when the property has a clear place for the unit, utilities are practical, and the owner has a realistic budget for the level of independence they want. A true rental-ready unit needs more than a bedroom and bath. It needs privacy, storage, durable finishes, a functional kitchen, safe access, and comfortable heating and ventilation.

Love Construction works with homeowners across Seattle, SeaTac, Burien, Renton, Bellevue, Kirkland, and King County on ADU and DADU construction, home additions, and home remodeling. If you are comparing options, start with the use case: family, rental, work, aging-in-place, or resale value.

FAQ: ADU Builders in Seattle

How much does an ADU cost in Seattle in 2026? Many Seattle ADU projects cost $150-$350 per square foot for planning purposes. Basement conversions can sit lower when the existing shell is usable. Detached backyard cottages often cost more because they need new foundation, utilities, roof, exterior envelope, drainage, and site work.

How long does it take to build an ADU in Seattle? A practical planning window is 6 to 12 months from early design through final inspection. Simple interior conversions can move faster, while detached ADUs, utility upgrades, structural engineering, and difficult sites can extend the schedule.

Is a detached ADU more expensive than an attached ADU? Usually, yes. A detached ADU needs more standalone construction: foundation, roof, siding, windows, insulation, heating, ventilation, sewer, water, electrical, and drainage. An attached ADU may reuse more of the existing home's structure and utilities, but older-home upgrades can still add cost.

Do Seattle ADUs need permits? Yes. ADUs and DADUs need permit review for building, zoning, utilities, safety, and site conditions. The exact path depends on whether the unit is attached, detached, inside existing space, or part of a larger remodel.

What should I ask an ADU builder before hiring them? Ask how they estimate utility work, permit timing, site access, structural upgrades, waterproofing, ventilation, insulation, and finish selections. A good builder should explain the budget range, the likely surprises, and whether an ADU, DADU, or addition is the best fit for the property.