The addition on the existing plan
A 30 × 24 ft house with a family room of 14 × 12 ft at the back. The new walls are red; the piece of the old back wall that opens up is yellow. The same plan gives the existing, the work and the new sheet.
How much square footage it adds
Under ANSI Z765 the finished area above grade is measured at the outside of the walls. The addition adds 174 sq ft to the home; inside its walls the family room is 155.3 sq ft.
Finished area under ANSI Z765
From the calculation core, the same plan before and after the work
| Plan | Square footage |
|---|---|
| Before the addition | 747 sq ft |
| After the addition | 921 sq ft |
| Added | 174 sq ft |
Distance to the lot line
How close an addition may come to the lot line is set by your town, not by a national rule: ask the zoning or building department. In the editor, the Plot tab shows the shortest distance from the house to the boundary on each side, on the plan and on the site plan of the export.
Finishing a basement
A basement is a level under Floor 1. Draw its rooms like any other floor; ANSI Z765 counts finished area below grade separately from the above-grade total.
How it works in the editor
- Draw your home as it is, or open an example home.
- Draw the walls of the addition. Choose each new wall and set Renovation to New: it turns red.
- Where the addition opens into the house, add an opening in the old exterior wall and set its Renovation to New: the piece of wall that goes turns yellow.
- The area counts the plan after the work. Under Plot, draw the lot line to see the distance from the house to each side, also on the site plan in the export.
Sources
- ANSI Z765-2021, Square Footage: Method for Calculating, Home Innovation Research Labs. Last checked on September 24, 2026.