Most real houses are planned from the outside in. The outline is set by a plot, a planning condition, an existing building, a budget or a roof, and the rooms have to be found inside it. That reverses the usual advice to start from what you need: here the total is fixed and what you need has to be negotiated against it.
Where the footprint comes from, and which edge it fixes
A footprint is fixed for one of a handful of reasons, and each one fixes a different line. That distinction is worth 400 mm a side, so settle it before you draw anything.
- An existing building fixes the inside face of the external walls. Measure between the plaster faces and you have the footprint directly.
- A planning limit or plot coverage rule fixes the outside face. The rooms stop one wall thickness inside it.
- A permitted extension usually fixes the outside face too, measured from the existing wall.
- A structural frame or a module (a 600 mm block module, a 3 m column bay, a prefabricated panel size) fixes the wall centrelines.
- A budget fixes the gross area, since cost is quoted per gross square metre, and the outside face follows from it.
- An existing roof fixes the outside of the walls that carry it.
1. Measure the footprint to the inside face
Rooms are measured between wall faces, so the area you can allocate is the area inside the external walls. Take an 8 × 12 m outline with 400 mm external walls, an ordinary insulated wall in most of Europe. The arithmetic looks like this:
| Line | Size | Area | Meaning |
|---|---|---|---|
| Outline, outside face of the external walls | 8.0 × 12.0 m | 96.0 m² | The number on the planning application. |
| Inside the external walls (400 mm) | 7.2 × 11.2 m | 80.6 m² | The footprint you can allocate. Draw the envelope here. |
| Internal walls, about 28 m of 120 mm partition | −3.4 m² | Taken off the rooms once walls are drawn. | |
| Net rooms and halls | ≈ 77 m² | What the rooms and circulation actually measure. |
Sixteen per cent of the outline has gone before a room is placed. Every fitting problem that follows is easier if that loss is taken at the start rather than discovered at the end, and the wall thickness guide explains why 400 mm is a fair figure to take it with.
2. Do the arithmetic before the drawing
Add up the room targets, add the circulation allowance and the internal walls, and compare the total with the area inside the envelope. This takes five minutes and tells you which of three situations you are in before you have drawn a single block. The same 80.6 m² footprint, planned two ways:
| Room | Two bedrooms | Three bedrooms |
|---|---|---|
| Living, dining and kitchen | 28 m² | 25 m² |
| Main bedroom | 13 m² | 12 m² |
| Bedroom 2 | 10 m² | 9.5 m² |
| Bedroom 3 | — | 8 m² |
| Bathroom | 4.5 m² | 4 m² |
| WC | 1.5 m² | 1.5 m² |
| Utility | 3.5 m² | 3 m² |
| Storage | 2 m² | 1.5 m² |
| Hall and landing | 8 m² | 9.5 m² |
| Rooms and halls | 70.5 m² | 74 m² |
| Internal walls | 3.4 m² | 3.8 m² |
| Allocated | 73.9 m² | 77.8 m² |
| Unassigned inside the envelope | 6.7 m² | 2.8 m² |
Both fit, and the table shows what each costs. The three-bedroom scheme buys its extra room by taking 3 m² from the living space, a little from every other room, and 1.5 m² more corridor to reach the third door, and it leaves under 3 m² of slack. The two-bedroom scheme has 6.7 m² to spend, which is a larger bathroom, a real store, or a living room that is 34 m² instead of 28. Neither is right; the point is that the choice is now visible as numbers. The allocation guide covers how the targets themselves are set.
The three situations: the total is well under the footprint, so the leftover needs a purpose (step 5); the total is close, so the arrangement has to be efficient; or the total is over, so the brief has to change (step 4). Only the second case is really a drawing problem. The other two are decisions.
3. Place the rooms in the right order
Inside a fixed outline the order of placement matters, because the last room placed gets whatever shape is left. Place the things that cannot move first and the things that can absorb change last.
- Fixed points. The entrance, an existing stair, the drain connection, a party wall, the side that faces the sun or the street. These are not choices.
- The largest room. The living space, on the best side. It is the room that a poor position damages most.
- The wet rooms, grouped. Bathroom, WC, utility and kitchen sharing one or two walls, near the drain. Scattering them costs pipe runs and, in a renovation, floor build-up.
- The bedrooms. Away from the entrance and the living noise, each with an external wall for a window.
- The circulation, as its own block. Draw the hall and landing as rooms with an area, at 900–1200 mm wide, and make sure they reach every door.
- Storage, in what remains. The odd corners a fixed outline always produces are exactly the size of a cupboard.
4. When it does not fit
If the arithmetic in step 2 comes out over, or the blocks refuse to close, the answer is almost never to trim every room by 10%. A house where every room is slightly too small fails in every room at once. Change one thing decisively instead, in roughly this order of preference:
- Shorten the corridor. Open the kitchen to the living space and let rooms open off it. Every metre of 1 m corridor removed is a square metre returned to a room.
- Combine service rooms. A WC inside the utility, a bathroom that also serves as the ensuite, a store that is the landing cupboard.
- Stack the wet rooms on one wall. It shortens the plumbing and usually the corridor too.
- Shrink the largest room first. Two square metres out of a 28 m² living room is barely visible. Two out of an 8 m² bedroom removes the wardrobe.
- Drop a room. A third bedroom that would be 6.5 m² is not a bedroom. Make it a study or give the area to the other two.
- Question the outline's shape, if that is negotiable. A deep plan wastes less external wall per square metre but leaves middle rooms without daylight; a narrow one lights every room but spends more on corridor. See room sizes and clearances for the window-area rule that limits a deep plan.
5. Give the leftover area a purpose
A fixed footprint almost always produces a remainder: the blocks do not tile the outline exactly, or the targets came to less than the area. That remainder has three legitimate uses and one illegitimate one.
- Storage. The most valuable use of an odd pocket. A 600 mm deep cupboard along a corridor wall is worth more than the 600 mm added to the corridor.
- Wider circulation. A 1200 mm hall instead of 900 mm changes how the house feels, and is the place to spend area if the rooms already meet their targets.
- Growing the neighbour. Give the pocket to the room next to it by moving one shared boundary. The living room is usually the right recipient.
- Pretending it is not there. This is the illegitimate one. A 400 mm strip behind a wardrobe or an unreachable notch beside the stair is area the house paid for and cannot use.
Slivers narrower than about 600 mm cannot be used for anything but a cupboard, and slivers narrower than 300 mm cannot be used at all. If the plan produces one, move the boundary that made it rather than living with it.
6. When a room ends up outside the footprint
It happens when a room is drawn to its target and the target is larger than the space left for it. The temptation is to bend the outline a little. Resist it, unless the outline really is negotiable: a room 600 mm over a planning limit is a refused application, and 600 mm past an existing wall is a new wall. Treat the overhang as a message from the arithmetic and go back to step 4.
How this works in Gridsnap
The Space tool's third mode draws an Envelope: a dashed outline with no body that marks the footprint the Spaces should fill. Space edges snap to it, and once it exists the Space Plan summary reports the area still Unassigned inside it and any Space area Outside envelope, so both the pocket and the overhang in the diagram above are reported as numbers next to the plan. The envelope does not clip, fill or resize anything; it is the region the arithmetic is measured against.
One practical point follows from how Build plan works: exterior walls are grown outward from the Space edges, so the envelope is the inside face of the external walls. If your limit is the outside face, draw the envelope inset by the external wall thickness (400 mm at the default Outer preset), which is step 1 of this page in one move. What Gridsnap does not do is fit the rooms for you: it reports what is unassigned and what is outside, and the arrangement stays yours.