Height is not linear, and this surprises people
A six-foot wall is not twice the price of a three-foot wall. It is considerably more, because at a certain height a wall stops being a landscape feature and becomes an engineered structure — deeper footing, geogrid reinforcement running back into the slope, and in some cases a stamped engineering design.
Where that line falls depends on your town, and the three we work in most set it three different ways. Yorktown publishes a building-permit threshold: a wall lower than 4.5 feet is exempt, and at 4.5 feet and above a permit is required — unless the wall supports a surcharge, such as a driveway or a structure bearing on the ground above it, in which case a permit is required at any height. The Town of Carmel requires walls over four feet to be shown on the plot plan with details, and walls over six feet to be certified by a New York State licensed structural engineer. Cortlandt sets no height in its building-permit rules at all, but requires any retaining wall with an exposed face over four feet to be approved by the Town Engineer for drainage adequacy — which catches people out, because it is filed among the fence and wall height regulations rather than with the permits.
This is worth knowing before you design, because a wall at four feet eight and a wall at four feet four are two different projects on paper. Sometimes stepping a tall wall into two shorter ones with a planted terrace between them costs less and looks better than one tall wall.
The drainage is not an upgrade
Saturated soil pushes roughly twice as hard on a wall as drained soil does. A wall built without a drainage path behind it is being asked to hold about double the load it was designed for, every time it rains — which is why the overwhelming majority of failed walls in this county failed for that reason and not because of the stone.
So a correctly built wall includes excavation behind the wall line as well as under it, clean drainage stone rather than the soil that came out, a perforated pipe at the base running to daylight, and filter fabric separating stone from soil so the drainage does not silt up and stop working in year four. That is material, labour and disposal, and it is a real part of the number.
An estimate that is meaningfully cheaper than the others is usually cheaper here. It is worth asking directly: what goes behind the wall, and where does the pipe come out?
What else moves it
Beyond height and drainage:
- Rock. Putnam and northern Westchester both have shallow ledge, and a wall trench that runs into it can turn a day of excavation into three.
- Access. Whether an excavator can reach the wall line, and where the spoil and the base stone can be staged.
- Material. Segmental block is the most predictable. Natural stone costs more in labour and often less in material, especially where the stone comes out of the excavation.
- Length and returns. Corners, steps and returns are slower than straight run.
- Removing what is there. Taking out a failed wall, and dealing with the soil behind it, is its own job before the new one starts.
- Watershed rules. Inside the New York City water-supply watershed a land disturbance of 5,000 square feet can trigger a stormwater plan, against an acre elsewhere in the state.
Why there is no number on this page
Because a price per linear foot without a site attached would be wrong for most people reading it. The same forty feet of wall costs very different amounts depending on whether it is three feet or six, whether the excavator can get to it, and whether there is rock eighteen inches down.
What we will tell you at the estimate is which side of the engineering threshold your wall is on, what we expect to find when we dig, and what happens to the price if we find rock — in writing, before anything starts.