Designing walkout basement lots
What is a walkout basement lot?
A walkout basement lot sits on a slope. The house is built across a split in the lot so the basement level opens out at grade on the low side — a door or patio straight onto the yard — instead of being buried.
From a grading point of view that means two platforms at different levels on the same lot, with a controlled step between them:
An upper platform carrying the main floor, and usually the driveway and road frontage
A lower platform at basement floor level, on the downhill side
A retaining wall or graded slope holding the difference between the two
A step height fixed by the design, rather than something Level AI is free to choose
The step is what you set up in the model. Everything below is about pinning that height and direction down so Level AI builds the lot around it.
Setting up the split
You model the split with a grade break that has a Minimum Height to Left set — a "fixed offset" grade break, where both the height of the step and the direction it falls matter.
Level AI then holds that height difference across the line, and builds it as a wall or as a graded slope depending on the grade break settings.
The same approach is used for a truck loading dock, where the fixed offset sets the dock depth instead of the basement height.
⚠️ Prerequisite — the lot must be a NORMAL parcel category and SINGLE FAMILY sub type.
Step 1 — Set the lot category
On the parcel, set Parcel Category to NORMAL and the Normal Parcel Sub Type to SINGLE FAMILY. See Parcels for the full list of categories.
Step 2 — Draw the grade break along the split
Select Grade Break from the Grading Draw / Convert menu on the Allsite ribbon, and draw the line along the top of the split, on the edge of the upper platform.
Three things matter about how you draw it:
Snap Direction. Set it so the symbology shows the grade break snapped down the slope, away from the upper platform. The wall or slope is then built below the line you drew, and the upper platform is held at it.
Left and right. "Left" is relative to the direction the line is drawn, and the line style shows which side is left. Draw the line so the side that drops away is the side you intend — or keep the line and use a negative height instead.
Extent. Run the grade break out to the lot boundary. If it stops short, the ground beside it is left too constrained to grade around the step.
Keep corners at 60° or greater. Sharper than that raises a warning, and below 10° the run is blocked — see Grade Breaks for the angle requirements.
Step 3 — Set the fixed offset
On the grade break, set:
Minimum Height to Left — the height of the split. Use a negative value if the drop is on the right of the line as you drew it.
Max Height — slightly above the minimum height. 4 in / 0.1 m is enough, and it gives Level AI room to iterate the design surface.
Minimum Height to Left is a hard constraint, so it can fight the rest of your grading and cause a runaway.
If a run comes back as a runaway, check three things in order: the direction of the line, the headroom between Minimum Height to Left and Max Height, and the lot Max Grade.
Step 4 — Add building pads above and below the split (optional)
For detailed grading, place a Building on the upper platform and a second one on the lower platform, one either side of the grade break. Level AI then designs a finished floor level for each pad rather than grading the lot as one surface.
Skip this step if you only need the lot split and are not modelling the house itself.
If you do not want a wall
Set Allow Wall to off on the grade break. That stops Level AI flipping this grade break to a wall where it clashes with something else on the site.
Turning Allow Wall off does not create a slope on its own — there also has to be room for one. Level AI builds a slope only where Max Batter Width × Max Batter Grade is greater than Max Height. So:
Set Max Batter Grade to the grade you want the slope built at.
Set Max Batter Width to at least Max Height ÷ Max Batter Grade. A 6 ft split at 33% needs more than 18 ft of width.
⚠️ Leave room for the slope.
If you added building pads above and below the split, the gap between the two pads must be at least as wide as the slope needs. Pads set closer than that will force the grade break back to a wall, or make the run non-viable.
Run Level AI
Run Level AI Prepare, then Level AI. Review the generated feature lines across the split: you should see the upper and lower platforms separated by a wall or a graded slope at the height you asked for, with the lot grading out to the boundary either side. Below is the resulting surface from the run.
Related
Grade Breaks — all grade break settings, the wall defaults inherited from the web, and the corner angle requirements
Grading Relationship — relationship types and a worked split-pad example
Parcels — parcel categories and lot grading settings
How to design a truck loading dock — the same fixed-offset grade break used for a truck bay