What the tool does
A feasibility study answers one question before design starts: how much can be built here. You enter the site as a rectangle or as corner points (or open a DXF), mark which boundaries are roads and how wide they are, and choose the zoning district. The tool works out the buildable outline at the top of each floor from setbacks and slant planes, keeps the ground floor within the coverage ratio, stacks floors until the floor area ratio, the height limit or the outline runs out, and shows the result in plan and in 3D with a check sheet that names the legal basis of every limit.
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Korea
Coverage and floor area ratio maximums for the 21 zoning districts come from articles 84 and 85 of the Enforcement Decree of the National Land Planning Act; enter the local ordinance values, which are usually lower. A road narrower than 4 m moves the building line back by half the shortfall and the strip is taken out of the site area (Building Act article 46). In residential districts the north daylight setback is applied to the mass: 1.5 m for parts up to 10 m and half the height above, and from 2026-11-12, under the amended article 61 of the Building Act, 5 m for parts between 10 m and 17 m; the review date picks the rule. A road on the north side moves the boundary to the far side of the road. Parking follows table 1 of the Enforcement Decree of the Parking Lot Act, with uses added to one decimal and rounded at 0.5, housing by exclusive area under article 27 of the housing construction standards, the stricter Seoul ordinance as an option, and accessible spaces. Landscaping follows article 27 of the Enforcement Decree of the Building Act; for sites of 300 m² and more the ordinance ratio is entered.
Japan
The designated floor area ratio is limited by the front road width below 12 m times 4/10 or 6/10 (article 52), and the coverage ratio gets 10 points for a fire-resistive building in a fire or quasi-fire zone and 10 for a designated corner lot, or no limit where article 53 says so. Road slant planes use the applicable distance and slope of table 3 with the setback relief, adjacent slant planes start at 20 m or 31 m, the north slant plane at 5 m or 10 m, and the low-rise districts have an absolute height of 10 m or 12 m. The shadow check calculates hours of shadow on the 1.5 m, 4 m or 6.5 m plane on the winter solstice between 8:00 and 16:00 true solar time and compares the longest shadow in the 5-10 m and beyond-10 m bands with the limits of table 4. The texts are those of the Building Standards Act on e-Gov.
Other places
Where no rules are built in, enter the coverage ratio, floor area ratio, height limit and any slant planes (a starting height at the road or lot boundary and a slope) from the local zoning code, and a parking ratio.
Area programme and shadows
List the rooms or units you need with their areas. The usable area divided by the efficiency gives the gross floor area to compare with the mass. The sun position uses the equations of the NOAA solar calculator; shadows are cast by each floor of the mass onto the measuring plane in 10-minute steps on a 1 m grid.
Files
- Check sheet (PDF): limits, plan values, verdicts and legal basis, with the plan and 3D views and the floor areas.
- Excel (XLSX): the check sheet with links to the statutes, floor areas with the floor area ratio as a formula, and the area programme with formulas.
- DXF: the site, the site after the building line and every floor outline on its own layer at its height. glTF: the mass for 3D software. JSON: the project.
Limits
The mass is a simple stack of floors inside the envelope, not a design. Sky factor relief, slant relief for two roads or open spaces, district plans, height districts and building agreements are not calculated, and north setbacks on non-convex sites are approximate. Local ordinances and the permit authority decide the final figures: have a licensed architect confirm them.
Last checked 2026-10-19