Accessory Dwelling Units (ADU), Structures, and Additions


Accessory Dwelling Units are becoming a popular addition for homeowners. The city of Eugene has been working with architects and builders to create a database of free and paid, pre-approved plans to get homeowners off on the right foot. As of 3/26/2023 it will require about $10,000 for permits and System Development Charges for a standard ADU. These SDCs are charged by the city to upgrade the utility capacity and infrastructure for the increased amount of demand and load.

An ADU can be built as a separate structure, a garage conversion, adding a second story onto a garage, or as an addition attached to the primary home.

Building a detached ADU is essentially the same process of building an entire new home from the ground up, so all of the same steps are required. An attached ADU has some benefits of the infrastructure already being there, but in many cases, more rework is need to be done with the primary home for adding utilities, insulation, access, etcetera.

Things you need:

  • Plans to submit to the city, including building plans, a site plot plan, stormwater management
  • General contractor or the building knowledge and resources to be the general contractor and project manager
  • Utilities such as water supply, sewer drain connection, stormwater management and electrical supply – this can mean modification to your existing supply to allow the increased power and plumbing demands
  • Foundation, typically a concrete perimeter foundation in our area, and possibly a slab on-grade or other
  • Strong structure of framing and sheathing engineered to support your local snow and wind loads.
  • Insulation and energy efficiency, with codes becoming more strict to include continuous insulation on the outside of your stud cavity to increase performance and further protect your home. Ceilings require R49 while floors require R38 for code minimum. While more costly initially, these requirements will pay themselves off, protect your investment, and increase comfort.
  • Water resistant barrier/house wrap, typically vapor permeable in our climate with a rainscreen gap between the siding and sheathing or continuous insulation to allow the house the dry
  • Doors and plenty of windows for natural light, with proper flashing details to keep water out
  • Durable siding, trim and paint or stain for most applications, to withstand the sun and moisture in our climate
  • Heating and cooling, a ductless heat pump/air conditioner unit is the go to for smaller spaces
  • A fully featured bathroom and kitchen, including shower, sink, toilet, range, dishwasher, exhausts and cabinets
  • A clean and cozy home with flooring, drywall, paint, and other interior paneling and creative storage areas to keep the house feeling clean and open.
  • Quality roof, preferably with large overhangs, gutters, and drainage system to direct water away from your home.
  • Finally some outdoor living space to such as a patio, deck, or balcony likely with some covered area, and a decorative fence, border, or private entryway to make the tenant feel at home and extend their space.


Home additions can be attached or detached. If you are looking to convert a garage or add an outbuilding as an extension of the house – i.e. bedroom or office and not an entire dwelling unit, you are instead looking at an addition. These are more limited in their use, as you are not typically allowed to include anything more than a bathroom. They are a cheaper and desirable option if you are looking to add extra space for your family who will share some of the primary home’s living, namely the kitchen and amenities. It is a wonderful thing to provide space for an aging parent, growing kids, a friend or family in need of housing, or to extend your personal living space.

Things you will need:

  • All of the same things will be required as building an ADU except for the bathroom and kitchen. For example, a simple bedroom or office could be added as a jut-off from the house into the backyard. This new area still requires a plan, foundation, minimum insulation, heating, framing, siding, roof, etc.

Accessory Structures

Sheds, outbuildings, barns, patio covers, greenhouses

Any structure under 200sqft, measured from the interior wall area, and under 10′ tall to the top plate, not used as a habitable area is exempt from building permits. This means you are allowed to build sheds and small outbuildings for storage and project space without a permit needed for the structure. It is also possible to add some limited plumbing (typically limited to 2 fixtures) if you need a sink in your potting shed, or a toilet in your art studio. The plumbing and electrical aspects will always require a permit, albeit a relatively simple process if you hire a certified electrician or plumber to execute all of the work to code.

These unpermitted accessory structures do not need to adhere to the same (sometimes any) building code, though of course, they should still be built strong and secure to create a safe and usable space.

Anything over 200sqft will require a permit to be obtained from the city and at that point building codes will be applied. If the space is a heated or cooled, a detached accessory structure that is not habitable, shall meet the following envelope requirements: Walls R-21, Roof R-49, Floors R-38.

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