A retaining wall holds back soil, and when the soil is wet it pushes hard. It is one of the items that can change your budget quickly, so ask about it before pricing starts.

Retaining walls are structural. A licensed engineer should design them, and this post is only a guide to the questions.

01Why they matter on a slope

Soil on a slope wants to move, especially in heavy rain. A wall designed for the soil, the height and the water keeps it where it belongs.

A wall built without design can fail, and failure affects your house and your neighbours.

Plan the wall early. Adding one late can force a change to the house layout. A wall is part of the structure, not decoration. Example: a homeowner who adds a wall after the house is built may find the new soil pressure cracks the garage slab. Why this works: designing it with the house lets the engineer size the wall and the footing together.

02What changes the cost

Height, length, the type of wall, the soil and drainage behind the wall all matter. A taller wall can cost much more than a short one.

Ask your engineer which type is suited to your site and why.

Ask for a written scope that says what the quote includes: excavation, footing, wall, backfill and drains. Two quotes for a wall can differ widely when their scopes are different. Example: one quote includes drains and backfill while the other lists only the wall itself. Why this works: a written scope lets you compare like with like, so the cheaper quote is not simply the one that left things out.

03Drainage is part of the wall

Water behind a wall increases the pressure on it. Ask how the design handles drainage, including weep holes and drains behind the wall.

A wall without drainage is a warning sign.

Ask who will inspect the wall during construction before it is covered by soil. Water is the force that usually pushes walls over. Example: after a long rain, weep holes that stay dry may mean the drain behind the wall is blocked. Why this works: clear drainage lowers the pressure on the wall, so the same wall lasts longer.

04Ask before you build

Ask who designs the wall, who checks it and who signs the plans. Ask whether a permit is needed.

Keep the drawings, since you may need them if you sell or extend.

Check the condition of walls after heavy rain and fix small cracks before they grow. Asking these questions early shows you who is really responsible. Example: if the mason says the wall needs no drawings, ask who will take responsibility if it moves. Why this works: a named designer and checker give you someone to call if something goes wrong.

05Watch it after rain

After the first heavy rains, look at the wall and the drains behind it. Report any cracks, leaning or water pooling to your engineer early. Small signs are easier and cheaper to fix than large ones. Example: a hairline crack that you photograph in October can be compared with the same spot in December. Why this works: a dated photo tells your engineer whether the crack is growing or settled.

06Questions for your engineer