This house isn’t a tiny house by any means. However, it seems that most people currently writing about tiny houses advocate for them as a living solution for 1 person, sometimes a couple, and rarely a couple with a kid. However, they don’t really expand on the concept of small living for bigger families. (Bigger families have a different dynamic than a family with one or fewer children – it’s not just a matter of square feet per body.) So, why 1000 square feet in particular? Actually, it has to do with our county’s code.
We live on Whidbey Island, in Washington, which is a bit north of Seattle, across the water from Everett, which is where the largest Boeing airplane factory is located. Most of the Seattle area has become heavily urbanized. However, Whidbey is connected to the mainland by ferry on the south end. (The north end of the island has a bridge, but the island is so long that by the time you get to where the bridge is, you’re no longer is the greater Seattle urbanized area.) Because of the ferry rather than a bridge, the island is surprising rural. You cross the ferry from the hectic, full streets of mainland Washington, and suddenly there are small, nearly empty rural roads, sparse housing, and quaint non-chain businesses. Whidbey is different than the surrounding areas. And the county planners want to keep it that way. So, they have codified a lot of principles into the zoning code to maintain the rural character of Island County. A lot of them have to do with strictly limiting development.
The county only allows one dwelling unit per parcel, with the exception that one may build a secondary house (commonly called a “Mother-in-Law Suite”, but the code calls it a “Guest Cottage”). That building is restricted to 1,000 square feet. (Code reference ICC 17.03.040) So, for this project for the time being, we’re limited to 1,000 square feet. Now, 1,000 square feet, unfortunately, means less than 1,000 square feet. Island County uses the 2012 International Building Code. The code says that the square footage is measured by the outside walls, not the inside walls. So, all the space occupied by the exterior walls is counted toward the square footage although you cannot live in the walls. This has the further disadvantage of reducing usable square footage even further for well-insulated walls. If you have a 40’x25′ house with 1-foot-thick walls, then the inside will be 38’x23′, or just 874 square feet. 126 square feet will be occupied by exterior walls.
For my own reference (and your if you interested in a similar project), the 2012 International Building Code defines Gross Floor Area as:
The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, closets, the thickness of interior walls, columns or other features. The floor area of a building, or portion thereof, not provided with surrounding exterior walls shall be the usable area under the horizontal projection of the roof or floor above. The gross floor area shall not include shafts with no openings or interior courts.
The Island County Code specifically modifies this definition as follows: (Code reference ICC 17.03.040)
Gross floor area means the total area in square feet of all floors of a structure measured from exterior walls, except that the following shall not be included in the calculation of gross floor area as it pertains to a structure:
a. Open courtyards, decks, porches and roof overhangs; and
b. Structures that provide spaces for mechanical equipment associated with a pump house, generator, hot water tank, heating systems, etc.; and
c. Any space where the floor to ceiling height is less than five (5) feet; and
d. Any space that meets the definition of a basement pursuant to the Uniform Building Code.
At the moment, I am looking at a house shell kit that has exterior walls of 39’7.5″ x 25’5.5″. That’s about 1008.786458 square feet. So, I’ll need to make a utility room (water heater, etc) of at least 9 square feet.
Note the thing about basements. The “Uniform Building Code” is now defunct (and has been for a while), but it has been superseded by the International Building Code. So, here’s the definition of “Basement”
BASEMENT. A story that is not a story above grade plane (see “Story above grade plane“). This definition of “Basement” does not apply to the provisions of Section 1612 for flood loads.
So, it’s a story that’s not a story above grade. So, what’s a story above grade?
STORY ABOVE GRADE PLANE. Any story having its finished floor surface entirely above grade plane, or in which the finished surface of the floor next above is:
- 1. More than 6 feet (1829 mm) above grade plane; or
2. More than 12 feet (3658 mm) above the finished ground level at any point.
And then we need to know what Grade Plane means
A reference plane representing the average of finished ground level adjoining the building at exterior walls. Where the finished ground level slopes away from the exterior walls, the reference plane shall be established by the lowest points within the area between the building and the lot line or, where the lot line is more than 6 feet (1829 mm) from the building, between the building and a point 6 feet (1829 mm) from the building.
So, the grade plane is the average height of the ground around the building. (Or if the ground slopes away from the building, you have to look at the lowest point within 6 feet of the building.)
So, if you have a daylight basement and 9 feet from basement floor to main floor and assuming a uniform slope down, then the grade plane would be 4.5 feet above the basement floor. So, a basement would certainly not be entirely above the grade plane, since it’s 4.5 feet under it. Therefore, we have to look at the other conditions. The next condition is if the next floor is 6 feet above grade plane. It is not, since it’s 4.5 feet above the grade plane. And the basement is not above finished ground level at any point, so it’s certainly not 12 feet above it. So, a daylight walkout basement meets the requirements of a basement.