Your yard steps up toward the back fence, right where the cottage would go. Something has to hold that soil in place. DADU retaining walls are one answer, and they can affect permits, engineering, drainage and where the cottage sits.

A DADU, or detached accessory dwelling unit, is a separate home on the same lot as a house. In Seattle, a low wall on an ordinary lot may skip a permit, but height, critical areas and nearby structures change that quickly.

Details below reflect official Seattle pages checked in October 2026. Confirm with city staff for your property.

Walls Versus Rockeries

A retaining wall holds soil at a steeper angle than the soil could hold alone. In a backyard, that might create a level pad for the cottage, a flat path or a patio terrace. The result is usable ground on a sloped strip.

Seattle treats rockeries differently. SDCI describes a rockery as placed rocks that control erosion, and the city does not count rockeries as retaining walls. Rockeries lean on firm, stable soil for part of their support, so true retaining walls face stricter structural standards.

Do Small Walls Need a Permit?

The SDCI retaining walls and rockeries page lists three conditions for skipping a construction permit:

• Height of 4 feet or less, measured from the bottom of the footing.
• Location on a parcel with no environmentally critical area.
• Low risk of damage to adjoining property or structures.

A wall that misses any one of those conditions needs a construction permit. SDCI notes that a wall at the property line may carry a risk of damage, so it needs a permit. A grading permit can apply as well, depending on how much soil moves.

When Do Engineers Get Involved?

Engineering needs vary with height, soil and loads. SDCI notes that drawings for rockeries and retaining walls commonly need a stamp from an architect or engineer. A wall holding up a driveway, slope or building carries far more pressure than a garden border.

Rockeries follow SDCI Tip 321. A rockery design must come from a geotechnical engineer or follow the city’s prescriptive standard. Any rockery over six feet tall, or one that departs from that standard, needs an engineered design.

Drainage Behind the Wall

Water is often the biggest threat. When soil behind a wall fills with water, pressure on the wall rises sharply. Nearly every wall design includes drainage.

Tip 321 shows the idea clearly. Its prescriptive rockery standard calls for at least 12 inches of gravel drainage material behind the rocks and a perforated drain pipe at least 4 inches across at the base. SDCI notes a side sewer permit is needed to connect wall drainage to a side sewer.

Property Lines and the Footprint

Walls near a boundary need extra care. SDCI says a wall or rockery built near a property line or the right-of-way must sit on the owner’s own property. A fence may not mark the true line, so a survey helps.

Walls and buildings affect each other. Tip 321 says no surcharge load, such as a building or driveway, may sit on a rockery or within a distance equal to its height. Lot coverage ignores these walls, which the post on Seattle lot coverage explains.

Common Wall Materials Near Cottages

Poured concrete walls suit taller grade changes and spots where a wall carries a building or driveway load. Segmental block walls stack interlocking units and appear often in yards. Timber walls cost less up front, though wood near soil tends to have a shorter life.

Rockeries use large placed stones and fit Seattle’s hillside look. Each material has its own drainage and footing needs. The designer picks one based on height, load, access, looks and budget for the specific lot.

Planning DADU retaining walls alongside the cottage lets one set of drawings cover both. Equipment that digs the foundation can often shape the wall footing in the same visit.

A Hypothetical Queen Anne Lot

Picture an invented example: a Queen Anne lot where the back third sits about five feet above the house. An old rockery holds the upper yard. The owners want the cottage on the upper level.

At that height, any new or rebuilt wall needs a permit. The cottage has to stay back from the rockery by at least its height. Nothing here describes a real property, but it shows how one wall can set the building location.

Common Wall Surprises

• Old rockeries that lean, bulge or seep water.
• Walls that sit partly on a neighbor’s lot.
• Drain lines with no approved outlet.
• Cottage footings too close to a rockery.
• Critical area mapping that removes the exemption.

Retaining Wall FAQ

Can an existing rockery stay?

Often it can, if it is sound and the cottage design respects it. Leaning, bulging or wet areas can signal trouble, but only a qualified professional can judge condition. The cottage still has to keep loads back from the rockery. Ask about the wall’s history and height early in design.

Do walls count toward lot coverage?

No. SDCI states that rockeries and retaining walls do not count toward the lot coverage limit. They still take up yard space and may affect where the cottage and paths fit. The post on Seattle ADU setbacks covers how the building relates to lot lines.

Is a low garden wall ever a problem?

Sometimes. A short wall on a parcel with a mapped critical area loses the exemption, and one at a property line may need a permit because of damage risk. Check the parcel’s ECA status and boundary before building even a modest wall.

Wall Planning Checklist

• Measure grade changes near the cottage site.
• Check the parcel for critical areas.
• Confirm the boundary with a survey.
• Note existing walls and their condition.
• Plan where wall drainage will discharge.

Settle wall height and location on the first site plan, and confirm whether a permit applies. For hillside lots, the post on the sloped lot DADU and the site preparation section go further.