Design Considerations

Some ADU design elements are regulated by the jurisdiction you live in and some are not. This section will outline the more significant design options that are available to you.

Where Can I Locate My ADU?

Generally, ADUs may be located inside the primary home, attached to it, or detached. Some cities do not currently allow detached ADUs. See the 'zoning information' pages for the options your city allows.

"Not all cities allow an ADU to be detached. Do you know if your city allows detached ADUs?"

This means the ADU is located in a structure on the property that is not attached to the main house. This can mean a new structure on the property, or it could be developed in a detached garage or other existing detached building on the property.

This means it could be built next to the main house in such a way that it is structurally attached. This could be a new addition to the house, or it could be an attached garage remodeled to include an ADU. In garage designs, some ADUs are built in the ground floor area of the garage, and some are built directly on top of the garage, preserving the ground floor area of the garage for parking (Note: be sure to check the 'zoning pages' to see if your city has any limitations on using garage space for an ADU.)

This means the ADU is developed inside the existing main house. It could be on the main floor of the house, in an upstairs area, or developed within a basement area.

NOTE: There may be special life-safety design considerations or requirements needed in your ADU, depending on how you want to locate or design it. Please be sure to discuss this with your building official.

What Are The Design Elements For ADUs?

Generally, all ADUs must meet some requirements in their design or layout. ADUs must have a bathroom, kitchen and sleeping facilities. However, there is other regulated design considerations, and a range of optional design considerations and features for you to think about. Some of these are listed below.

Design Considerations That May Be Regulated By The City Or Jurisdiction You Live In

Size Limits
The minimum and maximum amount of square footage allowed in your ADU will vary from city to city. At least one city includes decks in the allowable square footage. Some cities may also regulate the size of the ADU relative to the size of the primary home on the property. See the 'zoning information' profile page for the area you live in. (See sketches 2, 5, 8, 10 for examples.)
Sketch 2 Attached ADU Floor Plan
Sketch 5 Detached ADU Floor Plan
Sketch 8 Inside (basement) ADU Floor Plan
Sketch 10 Inside (daylight basement) ADU Floor Plan

Exterior Finish Materials
Some cities may regulate the look or finish of the materials you select for the exterior of your ADU. Generally, if exterior design is reviewed, the city will be evaluating the ADU appearance for consistency with the main house. See the 'zoning information' profile page for the area you live in. (See sketch 3.)
Sketch 3 Attached ADU Elevation

Many cities regulate the location of the exterior entry for an ADU. Some require the location of exterior doorways of the main house and ADU to not face the same street. Some allow the ADU entry on the front elevation, if it is screened from view. (See the 'zoning information' profile page for the area you live in.) Basement and upstairs ADUs may need special care in there exterior doorway designs. For example, basement areas may need to ensure that drainage from rain or irrigation, or other source of water, keeps water from pooling up or seeping into the basement ADU living area. Similarly, an outside stairwell to an upstairs ADU must be designed and constructed to be safe for both adults and children. (See sketches 6 and 7.)
Sketch 6 Detached ADU Elevation
Sketch 7 Inside (basement) ADU Site Plan

"Cities have regulations which affect how you can design your ADU. Be sure to understand what your city requires BEFORE you spend your money on design or materials."

Most cities require off street parking for your ADU. Sometimes this can be met with off-street parking you already have. If you plan on sharing existing parking, think about how your shared parking arrangement will affect you and your tenant's daily life. For example, if you plan to only use the parking apron in front of a two car garage, and you already have two cars, will the tenant's shared use of the apron area create garage access problems, or cause you to have to 'juggle' parking on the apron area. If you do not want to share your current parking areas with the ADU tenant, then you may need to provide a separate off-street parking area. See sketch 1 for an example of a driveway area which has been modified to allow a 'bump out' for parking. See the 'zoning information' profile page for the area you live in. (See sketches 1, 7 and 9.)
Sketch 1 Attached ADU Site Plan
Sketch 7 Inside (basement) ADU Site Plan
Sketch 9 Inside (daylight basement ) ADU Site Plan

There are some features that are required for the safety of occupants. If your ADU is attached to your main house, there will be fire separation requirements, which are aimed at reducing the spread of fire between the main house and the ADU (e.g. using 1-hour rated fire doors and wall materials between the ADU and main house). There will also be smoke detector requirements, which may also require constant electrical supply (not just batteries). There will be height and minimum openable area requirements for bedroom windows for emergency escape provisions (so that windows are not too high off the floor, or too small, to prevent individuals from being able to climb out of them.) Similarly, the ADU must have its own exterior door, and placed so that the occupant can exit directly to the outside in case of emergency, and not have to go through the main house. (See sketch 10.)
Sketch 10 Inside (daylight basement) ADU Floor Plan

Most jurisdictions require you to tap into the utilities that serve your main house to provide utility services to the ADU. This means you will need to decide where you will tap into your water and sewer, electrical or natural gas, and how you will provide phone and/or cable television services. The location of existing utilities on your property or in your main house will influence the cost of extending them to your ADU. You will want to consider whether the financial impact of these utility extensions will affect either the design or location of your ADU project. For example, if you are creating an ADU in existing basement space, is there already a bathroom available and, if not, how far and/or how difficult will it be to extend utilities. (NOTE: See the 'zoning information' profile page for the area you live in. If your city requires separate utility connections and services, then you will need to plan accordingly.)

Some cities may count the square foot area of outside decks as part of the total allowable floor area square footage of your ADU. Be sure to check on this before designing or adding in a deck to your ADU. (See sketch 2.)
Sketch 2 Attached ADU Floor Plan

At least one city does regulate the 'maximum' storage area that can be provided for or associated with an ADU. In practical terms, this probably won't be an issue that is regulated through your permit. However, you should provide some storage area for seasonal or other belongings, tools, and/or extra supplies.

"If I was planning my ADU project again, I'd spend a lot more time thinking through all my design options before I started construction. Believe me, it's well worth the time you spend doing so."

Design Considerations Not Regulated By Cities

Privacy can be an especially important issue with an ADU on your property.

A. If your ADU is inside your house or attached by a common wall: Noise carries through walls and can be a nuisance for you or your ADU tenant. You may want to consider insulating common walls between the ADU and the primary house. The insulation will act as a sound barrier. A well-insulated wall does not allow as much noise to be transmitted through it as does a non-insulated wall. For interior walls, an R-11 insulation value is sufficient and is generally regarded by ADU owners as money well spent. There are other ways to reduce noise. Consult your architect or contractor.

B. If your ADU is detached from your primary house: Privacy can be enhanced by giving consideration to the location of windows in your ADU, as well as to how you design and orient fencing, landscape and other screening materials.

Outside Space
You will need to decide which areas of outside property you will share with your ADU tenant, and which areas you will want to keep separated. Fencing or the use of landscaping as a screen can visually and functionally help you separate outside space. Also, if you are adding an addition to create an ADU, think about how the addition may block sunlight to the outdoor areas of your property, and to adjacent properties. (See sketch 4 for an outside space next to the ADU.)
Sketch 4 Detached ADU Site Plan

How will you provide for laundry facilities in your ADU. Most ADU owners either design the facilities within the ADU or allow the ADU tenant to share the laundry facilities in the main house. It is generally considered a negative if there are no on-site laundry facilities for the tenant, forcing them to seek off-site laundry facilities.

Electrical/Phone/Cable TV Infrastructure
One of the problems many rental tenants face these days is poorly located or inadequate provisions for electrical switches, plug-in electrical devices, telephone and computer plug-ins, and cable television outlets. Also, it is advisable to have separate heat source thermostat controls (that are independent from the main house). Make sure you have enough outlets on walls for the demands of the variety of electrical appliances found in modern households. Tenants will appreciate your thoughtfulness in designing these systems with them in mind. Careful consideration in the design stage will also help prevent requests for modifications to the systems by tenants, after they are in the ADU.

Even though the Puget Sound is a relatively temperate geographic area, we should all be aware of rising energy costs and their potential impacts to us all. Excessive heating and cooling costs can result from poorly insulated and weatherized ADUs. Similarly, heat build-up or cold air drafts can create uncomfortable living environments. Consider the amount of insulation value you will need to install in the floors, walls and ceiling to make the living environment comfortable. Also, don't forget to consider energy efficient doors and windows. Caulking and other weather-stripping techniques are also important. And just as importantly, make sure you have bathroom and kitchen fans ventilated to the outside, to properly vent (steam) moisture outside.

There may be other design considerations, depending on which city you are located in, or how you plan to locate your ADU. Thinking through design issues and options can save you time and money down the road, and increase the satisfaction with the unit by both the owner and tenant.