A floor plan is an allocation before it is a drawing. The house has a fixed number of square metres and every room wants more of them, so the real design decision is who gets what. Done as arithmetic first, that decision takes an evening. Done by redrawing walls until it feels right, it takes weeks and is still usually wrong about the hall.
1. Start from the right total
There are three different "areas" of a house and only one of them can be allocated. The gross area is the footprint measured around the outside of the external walls. The area inside the external walls is smaller by the wall thickness on every side. The net area is smaller again, by the internal walls. Rooms are measured net, so net is the number to divide.
Take a 10 × 10 m outline: 100 m² gross. With 400 mm external walls the inside measures 9.2 × 9.2 m, which is 84.6 m². About 30 m of 120 mm partitions take another 3.6 m². What is left to allocate is roughly 81 m², and a plan that hands out 100 m² of rooms on that footprint has already failed by 19% before anyone draws a wall. The wall thickness guide has the construction figures behind those numbers.
Rule of thumb: take 4% off the area inside the external walls for partitions in a house with ordinary rooms, 2–3% for an open plan with few internal walls, and 6% or more where masonry load-bearing walls run through the middle.
2. Set aside circulation before the rooms
The hall is the room everybody forgets to allocate and then finds they have drawn anyway, because the bedrooms have to be reached somehow. Plan for 10–15% of the net area in halls, landings and corridors. A corridor 1.0 m wide costs a square metre for every metre of its length, so a 6 m corridor is 6 m², or most of a small bedroom.
Under 10% is possible only when rooms open directly off the living space, which is what an open-plan house is really buying. Over 20% means the arrangement is wrong, not the rooms: a plan with bedrooms strung along a long corridor will never balance, however the room targets are trimmed. Corridors themselves should be 900 mm at minimum and 1000–1200 mm where doors open into them; the room sizes and clearances guide has the full set.
3. Divide the rest by group
Before deciding any single room, decide how much of the house each group of rooms gets. These shares are rules of thumb from ordinary family houses, not a standard, and the ranges overlap on purpose: a household that cooks and entertains moves five points from the bedrooms to the living group, and a household of six moves them the other way.
| Group | Share of net area | On 81 m² net | Note |
|---|---|---|---|
| Living, dining and kitchen | 35–40% | 28–32 m² | One open space or two or three rooms; the share is the same. |
| Bedrooms, all together | 30–35% | 24–28 m² | Three bedrooms share this; the main one takes the largest part. |
| Bathrooms and WC | 7–10% | 6–8 m² | A family bathroom, an ensuite or a WC, depending on the brief. |
| Utility, storage and plant | 5–8% | 4–6 m² | The first thing cut and the first thing missed. |
| Halls and landings | 10–15% | 8–12 m² | Under 10% needs an open plan; over 20% is a corridor house. |
The living group is deliberately the largest. It is the room that is used by everyone at the same time, so it is the room that fails first when it is short. Bedrooms are used by one or two people at a time and mostly asleep, which is why a 13 m² main bedroom is comfortable and a 13 m² living room is not.
4. Turn the shares into room targets
Now split each group into rooms, and check every room against two numbers: a working minimum area and a minimum clear width. A room that passes on area and fails on width is a strip, and a strip is not a room. Here is the 81 m² house from step 1 as a three-bedroom plan.
| Room | Target | Working minimum | What the target buys |
|---|---|---|---|
| Living, dining and kitchen | 30 m² | 10 m² living, 4.5 m² kitchen | Open plan; 2.4 m clear width where the sofa sits. |
| Main bedroom | 13 m² | 7 m² (11.5 m² for a double) | A double bed with 700 mm on both sides. |
| Bedroom 2 | 10 m² | 7 m² | A double bed with one side against the wall, or a single with a desk. |
| Bedroom 3 | 8 m² | 7 m² | A single bed, a wardrobe and a desk. Not a double. |
| Bathroom | 4.5 m² | 2 m² | A bath needs 1.7 m in one direction. |
| WC | 1.5 m² | 1 m² | 850 mm wide, door swinging out. |
| Utility | 3.5 m² | — | Washing machine, dryer, a sink, a cupboard. |
| Storage | 2 m² | — | Coats at the entrance, a linen cupboard, the vacuum cleaner. |
| Hall and landing | 8.5 m² | 900 mm wide | 10.5% of the net area. |
| Total | 81 m² | Net area inside the walls of a 10 × 10 m house. |
Notice what the table does not do. It does not give every bedroom the same area; it gives the third bedroom the minimum and the main bedroom the surplus, because equal bedrooms are a school, not a house. It does not round every room up to a comfortable figure, because the total would then be 95 m² on an 81 m² house. Targets are a budget. They only work if they add up.
5. Rebalance by moving shared boundaries
Once the rooms are drawn as blocks, the targets will not match. This is normal, and the way you fix it decides whether the plan stays coherent. Do not resize each room on its own. Find the boundary the two rooms share and move that one line: the area it sweeps leaves one room and arrives in the other, exactly, with no gap and no overlap left behind.
The arithmetic is simple enough to do in your head: area moved = length of the shared edge × distance moved. A long shared edge makes the boundary a sensitive control and a short one makes it coarse, which is worth knowing before you try to fix a 0.3 m² shortfall by moving a 6 m wall.
| Shared edge | Moved 100 mm | 200 mm | 300 mm | 500 mm |
|---|---|---|---|---|
| 3.0 m | 0.3 m² | 0.6 m² | 0.9 m² | 1.5 m² |
| 4.0 m | 0.4 m² | 0.8 m² | 1.2 m² | 2.0 m² |
| 5.0 m | 0.5 m² | 1.0 m² | 1.5 m² | 2.5 m² |
| 6.0 m | 0.6 m² | 1.2 m² | 1.8 m² | 3.0 m² |
Work through the plan in pairs. Living room short, hall generous: move their shared edge. Bathroom over target, bedroom 3 under: move that one. Each move fixes two rooms at once and leaves the total untouched, so after a handful of moves the whole plan balances rather than drifting off in one corner. When two rooms that need to trade do not share an edge, the arrangement is the problem, not the sizes.
6. Check proportion as well as area
The same 12 m² is a good bedroom at 3.2 × 3.75 m and a bad one at 2.4 × 5.0 m. Keep habitable rooms between square and about 1:1.5, and never let the short side drop below the working minimum: 2.0 m for a bedroom (2.4 m to walk round a double bed), 2.4 m for a living room, 1.7 m for a kitchen, 1.2 m for a bathroom. Moving a shared boundary changes proportion as well as area, so re-read the short side after every move.
Common mistakes
- Allocating from the gross area. The commonest error and the largest, worth 10–20% of the house. Allocate net.
- Leaving the hall to be whatever is left. It is never enough, and it is the room that then fails the 900 mm width.
- Giving the living room everything, then squeezing the bathrooms. A 2 m² bathroom does not get bigger because the living room is lovely.
- Resizing rooms individually. It leaves slivers between them and hides the trade-off. Move the shared boundary.
- Treating a target as pass or fail. A target is a budget line. Being 0.4 m² under it is information, not a defect.
- Balancing on area and ignoring width. A room that hits 10 m² at 2.0 m wide is a corridor with a bed in it.
How this works in Gridsnap
Gridsnap's Space tool is built around this method. Every Space carries a Target area, shows its Current area and the Difference, and adjacent Spaces snap edge to edge. The boundary two Spaces share gets a ⇔ handle: dragging it moves the line once for both rooms, and with a target set, the drag can snap at the point where that target is reached. Draw an Envelope for the footprint and the plan summary reports the area still unassigned inside it, so the leftover is a number rather than a guess.
What Gridsnap does not do is choose the allocation. The shares and targets on this page are yours to decide; the tool makes the arithmetic exact and the trade-offs visible while you do it. When the numbers balance, Build plan converts the arrangement into ordinary walls, and the rooms it detects will each come out slightly smaller than their Spaces, because real walls occupy floor. How to turn a space plan into a floor plan covers how much smaller, and why.