Buying a stock house plan usually means buying a licence to build it once, from drawings you may not change. Open House Plans is a free alternative for the first step: type "3 bed 2 bath farmhouse, 1,400 sq ft, porch" and it gives you a complete set of starting files, dedicated to the public domain, that you can take to a designer, an engineer or a builder.
Try it on the live page: <https://marisstella.app/open-house-plans/>.
Who made it. Like everything in this repository, it was built by an AI coding agent (Claude) in one session, at the request of the repository's owner, who asked for "a tool that can generate home designs from agent instructions and also generate actual residential home designs that are possible to construct. beautiful ones, cost effective ones, parameterized ones," as files "people can download and use freely".
Not advice. The plans are generated by software and marked not for construction. Nobody licensed has reviewed them. A real house needs a licensed review or your local code's prescriptive path, site and soils work, and a permit.
What you get
From a sentence or a JSON brief (bedrooms, baths, area, style, what to optimise for, garage, porch, foundation, lot, snow and wind), the generator returns fifteen files:
- a one-page presentation sheet, a dimensioned floor plan with furniture and fixtures, four elevations and a 3D view (SVG);
- the floor plan for CAD (DXF, on named layers);
- a BIM model (IFC4) with walls, slab, roof, windows and doors in openings that cut the walls, and every room as a named space with its area;
- the same model as OBJ for Blender or SketchUp;
- a bill of materials (studs, plates, headers, trusses, sheathing, roofing, concrete, insulation, drywall, fixtures), a window and door schedule, a room schedule and a cost breakdown (CSV);
- the whole design as JSON, a README with its checks, and the CC0 licence.
There are six styles (modern farmhouse, craftsman, modern shed roof, ranch, cottage and a budget minimal), studios to five bedrooms, and an accessible option with 36 inch doors, a wider hall and a bath with room to turn a wheelchair. Twelve ready-made designs are on the page.
The same brief always produces the same files, byte for byte, so a short JSON brief is a complete description of a house. That is what makes it useful to AI agents: an agent can turn a conversation into a brief, call the engine through the repository's command line or MCP server, show the person the result and the runners-up, adjust, and hand over the files. The agent guide is ventures/open-house-plans/AGENTS.md.
How a house gets laid out
The generator does not draw freehand. It treats a single-storey house as a rectangle on a 6 inch grid, split into zones along the street: an optional garage, a great room (living, kitchen, dining), and a bedroom wing with a hall and a row of rooms on each side, optionally ending in a primary suite. A split plan puts the suite on the other side of the great room. Small houses stack the bedroom and bath beside the great room instead.
Within that grammar it enumerates footprints, zone widths, hall positions, which rooms go on which side and in what order, suite sizes, kitchen sizes and sides. For a three-bedroom brief that is about 38,000 candidate layouts, searched in under half a second in a browser. Each one is scored on cost, how close it lands to the area you asked for, room sizes and proportions, plumbing spread, hall area, the front facade's symmetry and window rhythm, and daylight, weighted by whether you optimise for cost, balance or looks.
Then it does the engineering a designer would hand to an engineer: header, stud, joist and girder sizes from the National Design Specification's formulas (ported from this repository's open span tool and pinned to it by tests), and checks paraphrased from the residential code by section number: bedroom size, ceiling height, glazing, an emergency escape window in every bedroom, hall width, the egress door, and that every room can be reached without walking through a bedroom.
Does it produce real files?
Writing an IFC file by hand, with no library, is easy to get wrong, so the files were read back with independent open-source tools. For all twelve catalogue designs, IfcOpenShell 0.9 parsed every IFC with zero schema validation errors and its geometry kernel built every solid, 26 to 115 per house, with no failures; the rooms came back as spaces with the right names. The DXF files passed ezdxf's audit with zero errors. The output is in data/verify.txt.
What it says about cost
The cost estimate is anchored on one published number: the National Association of Home Builders' survey put the average 2024 construction cost at $428,215 for 2,647 square feet, about $162 a square foot, split over eight stages. Each stage is scaled by the design's own geometry: wall and roof area, footprint, perimeter, windows and doors, plumbing fixtures. How each stage splits over those drivers is an assumption, written out in cost.mjs. So the figures below are what the model says, in 2024 US-average dollars, not quotes. They come from data/study.txt.
Size moves cost per square foot most. A three-bedroom, two-bath house in the budget style models at $221 a square foot at 1,092 square feet and $162 at 2,176. The total still rises, from about $241,000 to $352,000, because the fixed costs (site work, the kitchen, the baths) are spread over more floor.
Style costs more than layout. For the same program and the same 50 x 28 ft footprint, the modern farmhouse (a steep 9:12 roof and a full-width porch) models at $214 a square foot and the budget minimal (a 4:12 roof and a small entry porch) at $196, about 9% apart. The full-width porch alone is about $19,000.
Layout barely moves the modelled cost. For each brief, the chosen layout costs within 2% per square foot of the median layout that passes all the rules. The search earns its keep on how the house lives (room sizes, plumbing, hall, facade), not on price. That is partly a blind spot of the model, which cannot see pipe-run length or how complex the partitions are.
Single choices on the farmhouse: a crawlspace instead of a slab adds 8.5%, a two-car garage 13.6%, premium finishes 7.7%; basic finishes save 3.3% and 8 ft ceilings instead of 9 ft save 0.9%. Advanced framing (2x6 studs at 24 inches) uses 12% to 15% less wall lumber than 2x4s at 16 inches on the same plans.
Those figures rest on assumptions as well as the anchor, so they were repriced under 500 random sets of the assumptions (data/sensitivity.txt). The size effect stayed between 1.32x and 1.44x, the farmhouse premium between 6.5% and 12.3%, the crawlspace between 5% and 11% and the garage between 11.6% and 14.5%. The porch was the least certain, at $10,500 to $28,200.
If you want a cheaper house, the model's advice is old advice: build smaller, keep the roof simple, and choose the porch and the garage on purpose.
What it cannot do yet
Every house is one storey on a rectangle: no stairs, L shapes or basements, and a five-bedroom with a garage comes out 98 feet long. The beauty terms are a designer's heuristics, not findings. Trusses, lateral bracing and foundations are listed for others to design. And the cost model has one anchor. The next steps are two storeys, L-shaped plans, a site plan from the lot, and having one catalogue design reviewed by a licensed professional.
The code is MIT-licensed and the plans are public domain. Start at the venture's README.