Accessory Dwelling Units

Healthy. Comfortable. Durable. Efficient.

Accessory Dwelling Units (ADUs) offer many exciting opportunities – from providing close (but not too close!) housing for extended family members to increasing options for living a car-free lifestyle to the prospect of an additional income stream for homeowners. Here is a short primer on ADU’s and a few tips for starting the process of building an ADU on your property.

What are ADU's & DADU's?

An ADU is a self-contained housing unit located on the same property as a single-family home, whether that’s a separate unit within the home, an addition attached to the home or a standalone structure. ADUs are sometimes referred to as “mother-in-law apartments,” “carriage houses,” or “granny flats.” These units include all of the facilities necessary for day-to-day living including a kitchen, sleeping area, bathroom and a separate entrance. A DADU is an ADU that is detached from the main dwelling (DADU=Detached ADU).

ADU code requirements

The ADU code requirements where updated in August 2023. For the most up-to-date information see the City of Bellingham website. Additionally, don’t be afraid to give the city planning department a call! They are happy to answer questions about your specific situation or if they can’t, to point you in the right direction!

There is a limit of two ADU’s per lot.
Currently, owner occupancy is required for properties in areas zoned residential single until occupancy requirements are preempted by state law (January 2026).
There is a size limit of 1000 square feet.
Generally, one on-site parking stall is required for an ADU. There are exceptions, find them here.

How much will an ADU cost?

There are three main factors that make up the cost of any building project, whether it’s a remodel, a new home or an addition: the permitting and utility connection costs, the complexity of the scope of work, and the cost of construction. When considering an ADU, perhaps the biggest factor that impacts cost is whether you plan to incorporate the ADU into your existing home (either by remodeling existing space, or as an addition) or whether you’d prefer to build a detached structure. Building a detached ADU is just like building a whole new house, only smaller. So, as a general guideline, detached ADUs are likely to be more expensive than an ADU within the existing home.

Permit costs

Homeowners pay for a building permit and an impact fee when building an ADU within Bellingham city limits. The cost of the building permit is based on the square footage of the project and the permit fee covers costs associated with inspecting the work and connecting to municipal services like the sewer The impact fee contributes to parks, transportation and schools, with the idea being that adding a housing unit will add residents to the area who will use these services. The city has a permit calculator to help you estimate the cost of permitting your ADU.

Scope of work

The scope of work includes the design of the ADU, the types of building materials you choose (e.g., roof, siding, windows, and doors) as well as the finishes (e.g., flooring, cabinets, plumbing fixtures, and lighting).

Even though ADUs are smaller than a full-sized home, they aren’t actually much simpler. This means working with a building designer or architect and an interior designer to help hone your vision into a buildable and permit-able set of plans. A good designer will help you effectively utilize the space inside the small footprint of an ADU, so we suggest seeking out a designer who is experienced with compact spaces.

You can make the biggest impact on the cost of your project by limiting the complexity of the design and carefully selecting building materials . Metal vs composite roofing, triple-pane vs standard vinyl windows, quartz vs laminate countertops, designer fittings vs off-the-shelf plumbing fixtures, custom cabinets vs IKEA – the list goes on and on. Your designer will help you weigh the pros and cons of these decisions and guide you in finding the right mix of materials that will be both durable and scaled to your budget.

Construction costs

It would be wonderful if custom construction projects could be priced by the square foot as a method for comparing builders or estimating total project costs. Unfortunately, design details, material selections, and site conditions play such a big role in determining costs, as do the different construction methods used by builders, that it’s nearly impossible to offer a square foot price that would prove useful to anyone who is in the process of planning a project.

What is possible though, is to share a ballpark cost for building an ADU. Before we get there, please keep in mind that an ADU squeezes all of the functions of a full home into a much smaller footprint! In general, we have found that a high-quality 1,000 square foot standalone ADU won’t cost less than $350,000. Costs will increase based on the complexity of the design, the material choices, construction methods, site conditions, and building performance details like additional insulation, energy efficient appliances or solar panels.

Getting started

When you’re ready to start planning your ADU, we suggest starting with the ADU code restrictions to determine if you can legally build an ADU. The city planning department and this building guide by Bellingham for Everyone are great resources for folks in the early planning stages of a project like this. Following that, get in touch! We’re happy to schedule a site visit and/or recommend a designer who is experienced with designing compact spaces. We love building ADUs and we’re excited to do our part to help increase housing options in Bellingham.