An open-plan living space is not one room whose size you look up. It is three activities sharing a floor, and the reason it works is that they share the circulation as well. Size it by adding the zones, and the total tells you what it can hold instead of the other way round.
Size it zone by zone
| Zone | Minimum | Comfortable | What decides it |
|---|---|---|---|
| Kitchen | 4.5 m² | 8–10 m² | One 600 mm run plus an aisle is the minimum. 8–10 m² is a kitchen two people can cook in. |
| Dining | 6 m² | 8–10 m² | A six-seat table is 1600 × 900 mm and wants 800 mm clear on every side. |
| Living | 10 m² | 14–18 m² | A sofa is 900 mm deep and the screen wants 2.4–4.0 m of viewing distance. |
| Route through | 1 m² | 2–3 m² | A clear path of 800–900 mm that does not cut between the sofa and the screen. |
Add the comfortable column and the total is 32 to 41 m². Add the minimums and it is 21.5. That range is the honest answer, and where in it your project sits depends on how many people eat at the table and whether two of them cook at once.
What each total holds
| Total | Suits | What you get |
|---|---|---|
| 22 m² | One or two people | Every zone at its minimum. Works, but the dining table is the circulation. |
| 26–28 m² | A couple, occasional guests | A real kitchen, a four-seat table and a two-seat sofa. |
| 30–35 m² | A family | The figure to design to. Six at the table, a full sofa, and a route past both. |
| 38 m²+ | Entertaining, an island | An island needs 1000 mm clear on every side before it earns its 2 m². |
In a whole house, the living, dining and kitchen group takes roughly 35 to 40 per cent of the net floor area. On a 100 m² house that is 28 to 32 m², which is exactly the range above, and it is the reason a family house below about 90 m² usually has to open the kitchen up rather than choosing to.
Depth decides more than area
The commonest failure in an open-plan room is not that it is too small. It is that it is too shallow, so the living zone cannot form. Work the depth out from the seating:
- A sofa is 900 mm deep.
- A screen wants 2.4 to 4.0 m of viewing distance.
- That is 3.3 m before a coffee table, a route or a wall thickness.
So the room needs about 3.4 m minimum and 4.0 m comfortably in the direction the seating faces. Below 3.4 m the sofa ends up against the same wall as the screen, at right angles to it, and the zone stops reading as a living room.
A long room is easier to zone than a square one. Zones fall naturally into a row along its length, each one getting the full depth, and the route runs down one side past all three. A square 32 m² room, 5.7 m each way, has to be zoned in quadrants, and the middle of it tends to become the crossroads nobody can furnish.
The route is a zone too
An open-plan room is usually the busiest floor in a house: it is the way to the garden, often the way to the hall, and sometimes the way to the stair. That route needs 800 to 900 mm of clear width, and it needs to run along one side rather than through the middle of anything.
Two rules keep it honest. It must not cut between the sofa and the screen, which turns the living zone into a thoroughfare. And it must not run through the kitchen work triangle, because the cook and the traffic then occupy the same square metre at the same moment. Both are avoided by putting the kitchen at one end rather than in the middle, which the worked plan above does.
Within the kitchen zone
The kitchen's own dimensions do not relax because the room is open. Base units are 600 mm deep, the aisle between facing runs wants 1050 mm minimum and 1200 mm to be comfortable, and there has to be a worktop landing beside both the hob and the sink. An island only earns its 2 m² when it has 1000 mm clear on every side, which is why islands need the 38 m² row of the table above rather than the 30 m² one.
Which shape the run should take is its own decision, worked through in the kitchen layouts guide.
When to keep the kitchen separate instead
Opening up is a trade, not an upgrade. What it buys is shared floor: the route and the dining space serve two zones at once, so the combined room does more with less area than three separate rooms would. What it costs is noise, cooking smells, and a kitchen that is permanently on view.
- Below about 26 m² the trade almost always favours opening up. Three small separate rooms at that total are three rooms that each fail.
- Between 26 and 40 m² it is a preference, and the deciding questions are usually acoustic: whether someone works or watches television while another cooks.
- Above about 40 m² a separate utility or scullery beside the open kitchen is often the better answer. The mess moves out of sight and the open room keeps its calm.
Extraction is the practical footnote. An open kitchen needs a real extractor ducted to the outside, not a recirculating hood, or the whole room smells of dinner the following morning.
Fitting it into the house
Give this room the best aspect in the plan. It is used by everyone at once and for most of the waking day, so it is the room that a poor orientation damages most, and it is the reason it goes on the drawing before the bedrooms do. Test the area against the whole-house budget rather than in isolation: the method is in how to allocate floor area between rooms, and the daylight rule of thumb of an eighth of the floor area means a 32 m² room wants about 4 m² of glazing.