How to calculate a house's usable floor area: PN-ISO 9836 step by step

01How to calculate it

The 2.20 m and 1.40 m height rule is set by the Polish regulation on the detailed scope and form of a building design (§ 20), which requires usable floor area to be calculated under PN-ISO 9836. This is how an architect calculates it in the building design, and it is usually the figure that house-plan catalogues give.

In short:

usable floor area = sum over the rooms of (area with headroom ≥ 2.20 m + 0.5 × area with headroom from 1.40 m to 2.20 m)

Step by step:

1. List the rooms on every storey: habitable rooms, the kitchen, bathrooms, corridors, walk-in wardrobes, utility rooms and a garage within the body of the house. Leave out balconies, terraces, loggias and unused lofts.
2. Measure each room at floor level, between the finished walls, that is, to the plaster. You do not deduct the skirting boards. If you measure the bare masonry, deduct the thickness of the plaster, which designs usually take as 1.5 cm on each face.
3. In the attic, mark two lines: where the headroom under the slope is 1.40 m and where it is 2.20 m. You measure the headroom vertically, from the finished floor to the finished ceiling.
4. Divide the floor into three bands: the band lower than 1.40 m counts at 0%, the band from 1.40 m to 2.20 m at 50%, and the rest at 100%.
5. Add up the rooms on each storey, then add up the storeys.

Where do the 1.40 m and 2.20 m lines fall? Under a straight slope, the distance from the knee wall is:

distance = (height of the line − height of the knee wall) ÷ tangent of the roof pitch

With a 1.00 m knee wall and a 45° roof (tan 45° = 1), the 1.40 m line lies 0.40 m from the knee wall and the 2.20 m line 1.20 m from it. With a 30° roof (tangent about 0.58), the same lines move out to about 0.69 and 2.08 m.

The most common mistakes are measuring along the outside of the walls (that is a different quantity: built-up area or gross floor area), adding in the balcony and the terrace, forgetting the plaster, and measuring the attic floor along the slope instead of horizontally. You can read more about attics alone in attic usable floor area: what counts at 100% and what at 50%.

02What counts and what does not

The rules in the table come from the regulation and from PN-ISO 9836. For the garage, the stairwell and the space under the stairs, architects' practice varies, so check how they were counted in the design you are comparing. Baukit counts them as in the table by default, and you can choose other rules in the „Area rules” window.

03Example: an attic with a 4 × 5 m room

Assumptions: an attic with a flat ceiling at 2.50 m. The knee wall is 1.00 m high and the roof pitch is 45°. Room dimensions are given to the plaster. The bedrooms and the bathroom run along the knee wall and are each 4.00 m deep, and the corridor runs under the flat ceiling. You can open the 4 × 5 m room from this example in Baukit.

1. The height lines. The 1.40 m line lies (1.40 m − 1.00 m) ÷ 1 = 0.40 m from the knee wall. The 2.20 m line lies (2.20 m − 1.00 m) ÷ 1 = 1.20 m from the knee wall.
2. The bands in the 4.00 × 5.00 m bedroom. The 0% band: 0.40 × 5.00 = 2.00 m². The 50% band: (1.20 − 0.40) × 5.00 = 4.00 m². The 100% band: (4.00 − 1.20) × 5.00 = 14.00 m².
3. The bedroom's usable floor area. 14.00 + 0.5 × 4.00 = 16.00 m² out of 20.00 m² of floor.
4. The other rooms are worked out the same way. Here, every metre of a room's length along the knee wall gives 3.2 m² of usable floor area out of 4.0 m² of floor.
5. The attic total.

In this attic, 8.8 m², about 17% of the floor, does not count towards the usable floor area. Once you add a ground floor, you add both storeys together. Ground-floor rooms with a flat ceiling above 2.20 m count in full.

How much difference does the plaster make? A room measured between the bare walls as 4.03 × 5.03 m has 20.27 m². Take off 1.5 cm of plaster on each face and you are left with 4.00 × 5.00 m, or 20.00 m². That is only 0.27 m² per room, but in a house with a dozen or more rooms the difference adds up to several square metres.

04How much floor area an attic loses

The result depends mainly on the height of the knee wall. Here is the same 4 × 5 m room (20 m² of floor, a flat ceiling at 2.50 m) with different knee walls and roof pitches:

In the free „PS.Arch 150” design from the catalogue of GUNB, the Polish building control authority (a house with an attic, 2023), the knee wall has a clear height of 0.81 m and the roof a pitch of 42°. The attic rooms there (without the stairwell) have 86.08 m² of floor, of which 63.81 m², or about 74%, counts towards the usable floor area.

05Work it out in Baukit

1. Click „Open in Baukit”. An attic opens with a 4 × 5 m room under a sloping roof.
2. Select the room. On the plan you will see the 1.40 m and 2.20 m lines and the bands counted at 100%, 50% and 0%.
3. The panel on the right shows the room's floor area and usable area.
4. Change the height of the knee wall or the roof pitch and watch the result change.
5. Press Esc so that nothing is selected. The notebook („Notebook” button in the bottom left-hand corner, or the 4 key) shows the usable floor area of the whole house on the Quantities tab.

Baukit works out the bands from the geometry of the roof, so it also copes with slopes on several sides of a room. The same drawing then works out the floors, the paint and the insulation.

06Frequently asked questions