We have a 1920’s craftsman is seattle. We are considering putting an apartment in the basement which is unfinished. It has tall ceilings and the foundation is such that it looks as though windows could be added to the side walls. What would be good questions / things to know to find out the feasibility?
An ADU designed and built by Simply Additions, a Connecticut contractor
Answer
Mia, in the City of Seattle what you would like to add to your home is called an “Accessory Dwelling Unit” or ADU for short. This is permitted outright in Seattle in owner occupied single family homes. The first part of “feasibility” is understanding the regulations that affect the creation of an ADU.
A partial list of the rules in Seattle is as follows:
- The home must be owner occupied.
- There can only be one ADU on the site.
- 1 off-street parking space must be provided for the ADU. This can be waved in certain circumstances.
- The ADU can be no larger than 1000 square feet.
- There can only be one entrance facing the street.
- Sleeping rooms need egress windows.
- The ADU must meet current codes.
- Electrical circuit breakers must be in a common area or located in the unit they serve.
As in all rules in Seattle there is fine print which can modify the above list but it would take forever to explain it. Seattle has a Client Assistance Memo on the subject (CMA #116a Establishing an Attached Accessory Dwelling Unit) which can be found here.
The next part of feasibility is understanding what improvements would be necessary.
Access
You probably will want to provide a separate access to the unit. If it’s a basement unit it may need a stair well dug and built to allow that access. You might think that the side yard would be a perfect place for it, but if you need to dig down more than 4′ you need room for a temporary slope cut between the well and your property line. An alternative to this is to get a temporary construction easement from a neighbor which would allow the excavation to project on to their property. The way to avoid this would be to put the entrance on the rear, where you probably have more room to make the excavation work.
Windows
Windows provide 2 functions - natural light & ventilation and emergency egress. You’ll need to provide 8% of the floor area in window space. Half of which must open. In addition, the sleeping rooms must have “egress” windows that have a NET CLEAR OPENING of at least 5.7 square feet, a clear opening height of at least 24″ and a clear opening width of at least 20″. Finally the sill of the window may be no more than 44″ above the floor. Generally a 5×4 slider, a 3×5 single hung or a 3×4 casement all fit the bill. This probably will result in the need for window wells. Bowman Kemp makes a prefabricated window well that meets the code. They can be found online at http://www.boman-kemp.com/products.html. The detail below is the one I generally use.
Detail of a window well that I use in my drawings
Seismic Improvements
If you’re working in the basement you may as well fix the single greatest weakness in homes of this era. That is the attachment of the structure to the foundation. This is where most houses fail in an earthquake - between the main floor and the foundation. While you’re working down there it is a relatively simple thing to upgrade the attachment. You should discuss the specifics with an architect or engineer on the site before you go too far down the road. In addition to this there are often “pony walls” between the foundation and the floor system. These are short walls that project up from the foundation to support the floor. Generally speaking these were not particularly well built back in the 20’s and at a minimum you should add a well nailed layer of plywood to the INSIDE of these walls to stiffen them up.
Insulation
You’ll want to insulate the new walls up to the current energy code, which requires R-19 insulation. This is most easily accomplished by adding a 2×6 furring wall inside of the foundation wall and putting all of the insulation and wiring in that wall. If this takes up too much space there are other alternatives but they generally are quite a bit more expensive. You may want to add some sound insulation to the ceiling as well.
Unit separation
Speaking of sound insulation, another step you can take to minimize the sound transmission between the floors is to use a double layer of sheetrock on the ceiling. I’d recommend 2 layers of 5/8″ Type “X” sheetrock, fastened to the resilient channels which are fastened to the joists. This not only provides fire separation between the units but it uses the same system of sound abatement employed in apartment buildings.
Floor Leveling
The chances are the old floor isn’t level. After you’ve installed the new pluming you can fix this by adding a layer of “self leveling underlayment”. An example of this is Gyp-Crete by Janes Brothers. It’s very easily installed and results in a flat floor that is suitable for tile or carpet installation.
Electrical Service
As mentioned in the rules above, the fuse box needs to be in a common area or it must be split up into two boxes, 1 for each unit. Since you’ll be re-doing the wiring downstairs anyway it should be relatively easy to put a supplemental fuse box in the new unit.
Those are the major issues associated with adding an ADU to an existing basement in Seattle. I’m happy to answer follow-up questions if you like. There is an exhaustive resource about ADUs in this area here.
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One Comment
good info.