A space plan is a set of lines with areas between them. A floor plan is a set of walls with rooms between them. The step from one to the other looks mechanical, and most of it is, but it contains two decisions that change every room in the house: how thick the walls are and which side of each line they sit on. Get those two right and the rest is drafting.
What changes at the transition
Every element of the space plan has a counterpart in the floor plan, and none of them is identical. This table is the whole transition in one place; the rest of the page is the order to do it in and the arithmetic behind the last row.
| In the space plan | In the floor plan |
|---|---|
| A boundary line between two spaces | A wall with thickness, centred on the line |
| A boundary marked open | No wall, or a wide opening in one |
| The footprint edge | The inside face of the external wall |
| Two spaces that touch | A door between them, placed later |
| The planned area of a space | The net area of the room, a few percent smaller |
| A corridor block | A hall with doors swinging into it |
1. Resolve every shared boundary
Before any wall is drawn, walk round every edge two spaces share and decide what it is. There are only two answers at this stage. A wall means the two rooms are separate, and a door will be placed in it later. Open means the two spaces are one room in construction terms, however they are labelled: a kitchen and a dining space, a living room and a hall in an open plan, a bedroom and its dressing area.
Two rules cover most cases. Keep an edge open only when the two activities tolerate each other's noise, smell and sightlines, which in practice means the living group and nothing else. Never leave a bedroom, bathroom, WC or utility open to anything. An edge you cannot decide is a sign that the arrangement itself is not settled, and it is cheaper to fix that now than after the walls are in. The planning guide has the checklist for a space plan that is ready.
2. Choose the wall thicknesses
Three thicknesses cover a house: an external wall, a load-bearing internal wall and a partition. Reasonable European defaults are 400 mm, 250 mm and 120 mm; the ranges are 300–500, 150–250 and 75–150 depending on construction and climate. Decide them now, from the build-up you intend to use, because they set the size of every room. The wall thickness guide shows where each number comes from, layer by layer.
If you do not yet know which internal walls carry load, draw them all as partitions and accept that one or two will thicken later. That is a 130 mm correction on a few walls, not a reason to delay the whole conversion.
3. Decide which side of the line each wall sits on
A planning line has no thickness; a wall does, so the wall has to sit somewhere relative to the line. There are two honest conventions and they apply to different walls.
Interior walls are centred on the line. The boundary between two spaces belongs to both of them, so the wall straddles it and each room gives up half its thickness. With a 120 mm partition that is 60 mm off each side along the whole shared edge. Centring keeps the plan symmetrical: no room is favoured, and a wall that is later thickened to load-bearing grows evenly into both.
External walls are grown outward. The footprint in the space plan was allocated as the area the rooms occupy, so it should become the inside face of the external wall, not its centre. The wall is added outside the line and the rooms along it keep their planned size. If the constraint was the outside face, as with a plot limit, the space plan should already have been drawn inset by the wall thickness, which is covered in planning inside a fixed footprint.
4. The arithmetic: how much each room loses
Take a room planned at 4.0 × 3.0 m, which is 12.0 m². What it measures once the walls are in depends on which walls surround it:
| Walls around the room | Net size | Net area | Loss |
|---|---|---|---|
| External walls on two sides, grown outward; 120 mm partitions centred on the other two | 3.94 × 2.94 m | 11.58 m² | 0.42 m² (3.5%) |
| 120 mm partitions centred on all four sides | 3.88 × 2.88 m | 11.17 m² | 0.83 m² (6.9%) |
| One 250 mm load-bearing wall, three 120 mm partitions | 3.88 × 2.82 m | 10.92 m² | 1.08 m² (9.0%) |
| External walls on two sides, but centred on the line instead of grown outward | 3.74 × 2.74 m | 10.25 m² | 1.75 m² (14.6%) |
The last row is the mistake to avoid. Centring an external wall on the footprint line costs each room along it 200 mm instead of nothing, and across a whole house the loss is large: a 7.2 × 11.2 m footprint becomes 6.8 × 10.8 m inside, which is 73.4 m² instead of 80.6, a loss of 7.2 m² that nobody allocated. The other rows are the ordinary cost of walls, and the space plan should have allowed for them: roughly 4% of the net area in a house with normal rooms, as described in the allocation guide.
Losses land unevenly. A small room with four partitions loses the most in proportion, so the room to re-check first is the smallest bedroom or the WC, where a few centimetres of width decide whether the door can still open past the pan.
5. Add the doors and windows
Every pair of spaces that touched and was kept as a wall now needs a door, and the space plan said nothing about where. Place it where the swing does the least damage: hinged on the side that puts the open leaf against a wall, with about 800 mm of clear floor on both sides and the 800–900 mm leaf width that lets furniture through. Doors into a corridor are the ones to check: three doors swinging into a 900 mm hall can block each other, and the fix is a wider hall or a sliding door, which is a space-plan change.
Windows go where the room needs light, which is decided by the room's use and orientation rather than by the plan geometry. As a check, glazing of about a tenth of the floor area is a reasonable minimum for a habitable room. The door sizes guide and room sizes and clearances carry the exact figures.
6. Re-measure and compare
With walls and doors in, measure every room net and set the result beside its planned area. Most rooms will be a few percent under, which is expected and fine. The rooms to act on are the ones that have crossed a working minimum: a bedroom that has dropped below 2.0 m clear width, a bathroom that no longer has 600 mm in front of the WC. Fix those by moving a wall, not by accepting the shortfall, and remember that moving one wall changes the room on the other side of it too.
If the comparison shows that several rooms are wrong in the same direction, the space plan was wrong, not the conversion. Go back to it, change the allocation, and convert again. That round trip is what the space plan is for, and it is far cheaper than dragging a dozen walls to the same effect.
7. Dimension and document
Now the drawing is a floor plan and gets the treatment a floor plan deserves: dimension strings between wall faces, room names and areas, door and window marks, furniture to prove the clearances, and a print scale. The floor plan drawing guide takes it from here.
Common mistakes
- Converting before the boundaries are settled. A boundary decided after the wall is drawn is a wall to delete or a door to move.
- Drawing walls as lines. A thin-line floor plan reports the space-plan areas as if they were real, and every room is overstated.
- Centring the external walls. Half a wall lost on every side of the house, silently.
- Forgetting the door swings. A room that met its target on area fails once the door needs 800 mm of floor inside it.
- Keeping both drawings live. After the walls exist, the walls are the plan. Editing the blocks as well means every change twice and two drawings that disagree.
How this works in Gridsnap
Build plan performs steps 3 and 4 in one action on the whole arrangement. Exterior boundaries become walls at the Outer preset (400 mm by default) grown outward, so the Space edge is the wall's inner face. Shared boundaries set to Wall, and those left as Auto, become walls at the Inner preset (120 mm by default) centred on the shared line. Open boundaries get no wall. The presets are the same drawing settings the wall tool uses, so a plan drawn to regional thicknesses builds to them. Gridsnap then detects ordinary rooms from the walls and names them after the Spaces they came from, and the whole build is one undo step.
The Spaces leave the canvas and the walls become the authoritative drawing, which is step 6's rule made structural: there is no second live model to keep in step. The Space Plan is kept as a read-only source. Selecting a built room shows a Planned as row with the planned and target areas and the difference, which is the comparison in step 6 without a calculator, and the Built from spaces control can view the source or reopen it as an editable variation for the round trip. Build plan places no doors, windows or furniture: steps 5 to 7 are the ordinary floor-plan workflow. If walls already stand on the floor, Build plan asks whether to build the draft as a design option or replace the current plan, and lists exactly what a replacement clears.