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Planning an ADU in the City of Seattle? Read this first.

Sep 23, 2020 | ADU, Civil Engineering, Land Use Planning, Permitting, Residential

In 2019, the City of Seattle enacted legislation that makes it easier for property owners to build accessory dwelling units (ADUs) in areas zoned for single-family residential use. The City recently launched the ADUniverse website, featuring a gallery of pre-approved building designs and a guide to the ADU permitting process. Residents of Seattle who would like to build an ADU must legally permit the unit, and these pre-approved designs help expedite that process. There is no doubt that this will be a great help to property owners who are trying to navigate the ADU process on their own. Property owners should also be aware that they will have to adhere to specific site requirements and consider the limitations of pre-approved plans before embarking on an ADU project. By engaging with a land-use planner and civil engineer early in the process, a property owner can be sure that ADU plans are realistic, achievable, and efficiently permittable. Here’s why:

Pre-approved designs will still require a custom site plan.

The gallery of pre-approved ADU designs encompasses a variety of styles and types of ADUs. While the building plans are pre-approved for construction, each property will need a site plan created that is specific to their site to receive a permit. A site plan is a of map of a property that designates locations of current structures and other physical features along with the location, size, and type of the new structure. This allows the City to review the site for drainage, zoning and site-specific features that could impact a new dwelling or the permitting process. The site plan will describe where and what kind of structure will be built, which makes it one of the most important parts of the process. An experienced land-use planner and civil engineer can assist you in determining the best course of action, as well as foresee potential permitting problems before you submit your plans.

Standard ADU plans have limitations.

The 10 standard ADU plans are a great place to start for planning an ADU, but they do not allow for very much flexibility to site constraints or specific needs. For example, the plans are limited to detached one- and two-family dwellings of three-stories or less. If you use pre-approved designs, you will work with the designer of the selected ADU plan, but if you are interested in a custom ADU, attached ADU, tiny house, or a structure taller than three-stories, you will need to engage with a design and engineering consultant team to create custom plans and submit for traditional permit approval. Our team of civil and structural engineers and land use planner can advise you on the zoning and site constraints as well as structure types that are possible on your specific site. If changes to existing plans are small, you may be able to include them in the standard plan and still benefit from expedited permitting. Allowed changes must be limited and reasonable in scope, examples include alternate roof lines, bay windows and variations in wall heights.

Permitting timelines are only an estimate.

  1. The best-case scenario for a pre-approved design with no site constraints is a 2-6-week permit window. If your site has environmentally critical areas, such as steep slopes or wetlands, or it requires tree removal, it is likely that the permit will take longer. It is also possible that additional consultants will be required to evaluate the site. Engaging with a knowledgeable land use planner and civil engineer early in the process can help you anticipate these obstacles and help you choose the ideal location for the ADU on your property to mitigate site constraints. They can also work with the property owner to quickly hire and coordinate additional consultants and reports, if needed. This can be completed concurrent with site plan development.

The availability of pre-approved ADU plans will help property owners expedite the building design phase. It is important to also engage the right consultants for the site plan phase of the project. CG Engineering has extensive experience permitting residential and commercial structures within the City of Seattle. Our team can help you with structural modifications, site plans, consultant coordination and permitting of your ADU in the City of Seattle.

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