Garden walls and raised beds, for beds that hold soil

Not for a normal raised bed. A retaining wall is exempt from a construction permit when it is four feet or lower measured from the bottom of the footing, outside an Environmentally Critical Area, and causing no damage to adjoining property — and garden walls are usually one to three feet. The two things that catch people: the measurement is from the footing rather than from the ground you see, so a tall bed on a slope can be over four feet on the downhill side, and inside an ECA the exemption does not apply at any height.

Eighteen to twenty-four inches for general planting, which is high enough to work at without bending double. Thirty inches is the comfortable standing height if the point is to avoid kneeling entirely, and it roughly doubles the soil volume and therefore the fill cost. For vegetables, twelve to eighteen is plenty of root depth as long as what is underneath drains. Deeper is not better; it is just more soil to buy.

Wet soil pushing, with nowhere for the water to go. Saturated soil weighs far more than dry soil and exerts a lot of lateral force on a wall that was built as an edging rather than as a structure. Add deliberate watering on top of Seattle rainfall and a bed wall sees more water than most retaining walls do. The fix is the same as for any retaining wall: free-draining material behind the face, geotextile keeping soil out of it, and a route for water to leave.

Timber is cheapest and looks right in an informal or productive garden, but even cedar and treated sleepers have a life in this climate — plan on fifteen years or so for the course sitting in wet ground. Block is the value option that lasts indefinitely and comes in enough finishes to look deliberate. Natural stone costs roughly double block and is the one that still looks good in thirty years. All three need the same drainage; the material choice is aesthetic and budgetary rather than structural.

What is behind it. A garden wall holds soil, so it is a low retaining wall with the drainage that implies. A seat wall is freestanding and built to a specific height to sit on. The two get confused because they look similar from the front and are often the same height — but a seat wall built to hold a bed will bow, and a bed wall built at seating height without a comfortable cap is unpleasant to sit on. See our seat walls page if seating is the point.

Yes, and it is one of the better things to do with a slope. Terracing turns an unplantable bank into a series of level beds, each wall holding the one above. Two or three shorter walls almost always cost less than one tall one, and they avoid the engineering and permit that a single wall over four feet triggers. The thing to get right is drainage between the terraces, because water moving down the slope now has walls in its way.