An ADU is a permitted second home with a complete kitchen, built on a single family home site. There are specific regulations that control the size and sometimes the use of an ADU.

An outbuilding is a structure built on a single family home site that is detached from the primary structure. It cannot have a complete kitchen. (It cannot contain any cooking appliances other than a microwave oven). Outbuildings have specific regulations and restrictions that differ from community to community.

An outbuilding that is attached to the primary dwelling in any way (an interior hallway, covered breezeway or deck) is considered part of the primary dwelling and must not violate any local zoning regulations that are in affect when it is built at the time as the primary home or as an addition.

What category of building will you be planning and what are the key considerations in planning an ADU or away-space?

1. Research the regulations

Most communities' zoning and building departments publish their regulations online in some searchable form. The regs must be reviewed and understood as the first step in planning the structure. I do this research for my clients if I am engaged early enough in the process.

Research the jurisdiction that controls your site. Your postal address could be Bellevue, but the jurisdiction could be King County. Have your tax number or parcel number handy, as regs often vary by zoning category (e.g. R1, R2, R10) The ADU or outbuilding regs may limit the building size, what spaces are included in the size calculation, and the allowable uses of the completed structure.

The zoning regulations regarding lot line setbacks, height limitations and lit coverage maximum will all be listed in residential zoning regulations, and the ADU, like the primary home, must comply to those as well. Outbuildings will likely be in their own category in the residential area of the regulations.

(I highly recommend a more efficient and thorough route: Engage Warmmodern Living to design your structure and Lindal to produce it at the outset of the project, and we’ll do the research. Lindal has the best design process available to you, bar none. It will get you to the custom design of your lifetime, married to your site and respectful of your budget for as little as $2.00 per square foot!)

2. Waste not, want not.

If your site is served by septic system, that system was designed for a specific number of bedrooms. If you are planning a structure with a bedroom (any room with a doorway less than six feet wide near a full bath will be so defined), the added bedroom(s) may throw you over your system’s design capacity. Without writing a book here, suffice to say that triggering this condition may require an entirely new system built at today's standard, which is likely to be larger, have more expansion capacity etc. etc. than your system. At best, you need to budget for a new system. At the worst, you may be denied permits for a structure with a bedroom (even if you really don't intend the space to be a bedroom. (Go back a re-read the paragraph above this.)

3. Connect with the natural environment on a daily basis

Planning an ADU or outbuilding demands the same care to be in harmony with its natural environment as the primary residence did when you planned it. Thinking of structure, as less important than the primary structure leads to less excellent results.

We’ll study:

The topography and how it will affect the design;
The path of the sun and natural daylighting (Where do you want the sun in the morning/ the late afternoon?);
Trees that will shelter you from drifting snow, road noise, or shade you from intense summer afternoon sun;
Views, large and small;
Think of the exterior as a terrific extension of the interior, especially if the interior is limited to 500 or 800 square feet!

4. Disconnect from your family and guests

This project is about privacy; yours and others’. You want weekend guests to have quiet time apart from you; your grads to play their music without your hearing it; your wife needs to sleep during the day after being in the ER for 24 hours and you have a house full of active toddlers; or perhaps you work from home and regularly meet with clients (whom you’d prefer not to have walking through your home).

I’ve heard these and dozens of other reasons to plan an away space in the last several years. Our challenge is to anticipate the many uses of the cherished retreat and design for the kind of privacy that each requires. For example:

Having the space be semi-attached or detached from the main house facilitates acoustic privacy.
Planning it so that you cannot look into the retreat from the primary home, and they cannot see into main house from the ADU.
Away spaces should have their own private entrances from outside. Young people in particular like to come and go without being scrutinized.
Consider a small private deck where others can enjoy the Sunday paper before getting dressed.... or entertain quests without including you.

5. Include thoughtful amenities

And I’m not talking hand soap.

Think of every likely use and plan the space for it early. Foresight is typically better executed than afterthought .

Include something that will make each activity memorable and a credit to you. These might include one or two of the following:

A high-quality media center
A powerful exhaust unit to eliminate cigar odors after man cave football evenings
An ADA compliant bathroom for aging in place
Give them control: plan exterior lighting of patios and entry controlled inside the away place. The same for heat and air conditioning.
A piece of exercise equipment; a wine cooler, a coffee maker, a well-lit desk with basic supplies, room darkening shades, a wood burning fireplace etc. etc. etc.

6. Your evolving lifestyle

A young couple with whom I worked had what I’ll call a “fifteen year plan”. A large foyer and gallery would separate the main house (to the right) from the ADU (to the left). See the image above.

We discussed an ADU that would evolve in three phases as the family’s lifestyle evolved:

Phase 1. Initially the 600 square foot “away space' would be a full apartment for his elderly parents, who visit for the three summer months and the year-end holidays. A living room kitchen area, a bedroom with a large walk -in closet and ADA compliant bath.

Phase 2. The suite will become the office of his financial planning practice. A waiting room/assistant’s office with food prep area, his private office with a full bath and file room.

Phase 3. Finally when the couple feels that they no longer need to sleep on the upper level of the main house with their children, the away space will become a master bedroom suite: a sitting room, with refreshment center and a large bedroom with walk-in closet and full bath.

Together my clients and I worked successfully to create an away space that could be repurposed as described without changing the structure as built, except perhaps for a paint job and new flooring.

7. What “makes” a window is a wall

I have seen time and time again that providing opportunity for privacy in human-scaled away spaces is truly appreciated by family and guests alike and that the private time actually enhances the communal time. That leads me to say that “the best thing about being together is the ability to be apart.”

A Caution about Cost

Many people think that a small cottage or an ADU is a low cost idea… that a 1000 SF ADU costs one-third of a 3000 SF house. As with German cars, couture, even Cheerios, that is not the case. A small house has proportionately more of the same cost centers - a fully equipped kitchen, a heating system, two bathrooms - than larger home and proportionately less “empty space.” Experience shows that a 1000 square foot ADU can cost $50 to $75 per square foot more than a 2500-3500 square foot home built on the same site.