What’s an Accessory Dwelling Unit (ADU) Cottage?

I recently had the opportunity to design an Accessory Dwelling Unit (an ADU) for a client who was building the ADU for his 78 year old mother. Where I practice (the Seattle area), many municipalities are allowing ADU’s to be built as part of an existing residential unit – attached to the main house or as a detached unit.

My client contacted me to design this unit for his mother and this ADU / cottage will be the first unit in a 10-12 unit cottage project that he hopes to develop for this site in the future.

In our area, most ADU’s are built as attached units, however more detached ADU’s are being built as a detached “mother-in-law” apartment and are located in the back or side yard depending upon the local zoning codes.

Most cities allow only attached ADU’s because they wish to maintain the single-family house character of the neighborhoods. In municipalities where detached ADU’s are allowed, the zoning code usually requires the ADU to be built at the rear of the main house. Detached ADU’s are more expensive to build, however they do provide privacy for a homeowner who wants an ADU with the rental income but prefers not to have the tenant living in the same physical structure. Detached ADU’s in urban neighborhoods are usually built above a new or existing garage at the side or rear of the main house.

Zoning codes usually have a maximum size for the ADU – in this case, the area of the ADU cannot exceed 40% of the area of the main residence. For this project, the ADU had to be 739 s.f., or less, which is about the same area as a one bedroom apartment.

Attached or detached ADU? If the zoning code allows either, the deciding factor is often the design and layout of the main house and how an attached addition would affect the layout and appearance of the main house. If the lot size allows it, a detached ADU offers more design options and can have the character and appeal found in cottage homes.

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