Designing a contemporary home on a steeply sloping site in Seattle

SW Manning Street slopes significantly

Steeply sloping lots pose serious design challenges for Seattle architects. This is the 2nd article in a series where I describe how we dealt with those challenges on a home I designed on SW Manning Street in West Seattle.

Let’s recap. The lot slopes steeply down from the street (front to back) and the street slopes significantly (side to side). The view is to the north and east although a little south of east is the West Seattle Bridge, which we don’t really want to look at. We decided for a variety of reasons discussed in the earlier article to place the entry on the downhill side of the lot.

We want the light but we don't want to look at the West Seattle Bridge

How do you deal with a garage on a sloping lot?

Usually when the lot slopes down from the street and the street slopes along its length I start by placing the garage on the low side of the lot. In this case we’ve decided to place the garage on the high side in order to accommodate the entry. Once we’ve established that many parts of the design start to fall into place.

Architects like to avoid steep driveways

I try to design driveways so that they have no more than 10% slope. In this situation we find the actual elevation of the curb at the street in the center of the driveway and calculate our garage floor height from there. The road itself slopes greater than 10% so had we sloped the middle of the driveway down 10% the uphill side would be quite a bit steeper. Here we chose to make the center of the driveway roughly level. You can see all of this in the photograph above.

Setting the finished floor elevation

This puts the garage finished floor elevation at the garage door a couple inches above the existing elevation at the curb. This is where the biggest challenge lies. The garage floor is now from 4 to 13’ above the existing grade.

How to build the garage floor

In this case we had 2 choices.

Build huge retaining walls and place nearly 400 yards of fill in order to pour a concrete slab floor
Use a framed floor and pour a concrete topping slab over that.

There are other choices but we generally don’t use them in single family construction. We chose to build a wood frame floor. This resulted in either a tall crawl space or some potential living area.

Living area below the garage

It requires very little additional effort to convert that crawl space into living space so we chose that option. The ceilings in that area are a little lower than the rest due to the increased depth of the garage floor system.

The structure steps down the hillside

Terracing the lower floors

One of the architect’s primary goals when designing a site like this is minimizing excavation. For this reason the lower floors step up the slope in a terraced fashion. The lowest floor is smaller than the next floor up which is smaller than the main floor.

I don’t want to feel buried

The way to make these lower floors feel like they’re above ground is to put plenty of windows in the parts that are above ground. We designed the home so that the rooms without windows didn’t need windows. For example, a bath room, utility room, storage and wine room all can be underground without feeling awkward. The bedrooms, master bathroom and living spaces all get lots of windows and don’t feel as if they’re below grade.