Measuring gross internal area against the space standard
Gross internal area decides whether a unit passes the space standard. Stairs, sloping ceilings and built-in storage are where schemes gain and lose square metres.
Gross internal area is the number that decides whether a unit passes the nationally described space standard, and it is measured to the internal face of the perimeter walls. It includes internal partitions, it includes the stair at each floor it passes through, and it excludes any floor area with a headroom below 1.5 metres except under the stair. Getting gross internal area measurement space standard checks right at the drawing stage saves the argument at validation, because a unit that misses by half a square metre fails.
The rules are short. The disputes are all about the same four things, and they come up on every conversion.
What gross internal area includes and excludes
Measure to the inside face of the external walls at each level. Internal walls and partitions are included, as are columns, piers, chimney breasts, ducts and service risers within the dwelling.
Excluded are open external balconies, terraces, external stores accessed only from outside, and any part of a communal area serving more than one dwelling.
The rule that catches people is headroom. Floor area where the ceiling height is under 1.5 metres does not count. On a rectangular flat in a new block this makes no difference. On a top floor conversion under a pitched roof it can remove five square metres from a unit that looks generous on plan.
Floor to ceiling height is a separate requirement. The standard expects a minimum of 2.3 metres for at least 75 per cent of the gross internal area, and authorities apply it strictly on conversions of older buildings where floor build-ups eat into the original height.
Stairs, voids and sloping ceilings
The stair is counted at each level it passes through. So a two-storey maisonette counts the stair footprint twice, once on each floor, which surprises people who assume it counts once.
Voids do not count. A double-height living space counts the floor area once, not twice, and a stairwell opening on the upper floor is a void rather than floor area.
Under the stair is the exception to the headroom rule. Area beneath a stair is included even where the headroom drops below 1.5 metres, which is one of the few places the standard gives something back.
For a room in the roof, draw the 1.5 metre line on the section, project it onto the plan, and measure the area on the habitable side of it. Doing this properly at concept stage changes how the dormers are positioned.
Built-in storage and how it counts
Built-in storage is required in addition to the general internal area, and the minimum rises with the size of the dwelling. A one bedroom one person unit needs 1.0 m square, a one bedroom two person unit 1.5 m square, and the requirement increases for two, three and four bedroom homes.
Storage counts within the gross internal area. Where more storage is provided than the minimum, the surplus can count towards the general area requirement, provided it is at least 900 mm wide and reached from a habitable room or hallway rather than from outside.
What does not count is a cupboard housing the hot water cylinder, the consumer unit or the heat interface unit where it is fully occupied by equipment. Draw the storage cupboards with their internal dimensions on the plan, and label the area.
An illustrative one bedroom flat calculation
This example is illustrative. A first floor unit in a converted commercial building is drawn with a living and kitchen space of 24.0 m square, a double bedroom of 12.2 m square, a shower room of 3.6 m square, a hallway of 6.8 m square and a storage cupboard of 1.6 m square. Internal partitions account for a further 1.9 m square.
| Element | Area |
|---|---|
| Living and kitchen | 24.0 m square |
| Bedroom | 12.2 m square |
| Shower room | 3.6 m square |
| Hallway | 6.8 m square |
| Storage cupboard | 1.6 m square |
| Internal partitions | 1.9 m square |
| Total gross internal area | 50.1 m square |
Against a requirement of 50 m square for a one bedroom two person dwelling, the unit passes by 0.1 m square. The bedroom exceeds the 11.5 m square minimum and its narrowest dimension is 2.9 metres, so it passes on width. The storage cupboard exceeds the 1.5 m square requirement.
That is a scheme with no margin, and any change during technical design will break it.
The same flat with a reduced ceiling height
Now put the same layout on the top floor, under a mansard, with the ceiling sloping over the outer 1.4 metres of the living space and the bedroom.
Area under 1.5 metres of headroom is removed. In the living space that removes roughly 4.6 m square and in the bedroom roughly 2.3 m square. Gross internal area falls to about 43.2 m square and the bedroom drops to 9.9 m square.
The unit now fails on two counts, the overall area and the bedroom area. Nothing has changed about the building, only the roof profile above it.
The fix is either a larger footprint, a change in the roof form to raise the ceiling over the critical zone, or accepting one fewer unit in the building. All three cost money, which is why the measurement belongs at the feasibility stage rather than at planning.
Where a scheme fails on a single room
Bedroom width is the most common single-room failure. A room of 11.6 m square measuring 2.6 metres by 4.5 metres meets the area requirement for a double and fails the 2.75 metre width requirement.
Subdivided rooms in conversions produce this shape repeatedly, because the original window positions dictate where the partition can go. Moving the partition 150 mm is usually possible, and is usually easier before the application than after.
Single bedrooms have their own trap. At 7.5 m square with a 2.15 metre minimum width, a room drawn at 7.6 m square but 2.0 metres wide fails, and it will be described in the officer's report as a room that cannot accommodate a bed and a wardrobe.
Presenting the schedule of accommodation
Give the authority a table rather than making them measure. List every unit by reference, its type expressed as bedrooms and bedspaces, its required area, its proposed gross internal area, the bedroom areas and widths, and the storage provision.
Add a note on how the areas were measured, including the treatment of headroom and the stair. Where any unit passes by a small margin, say so rather than leaving the officer to find it.
A Planning Statement that carries the schedule and explains the measurement basis answers the question directly, and on a scheme where the layout is driven by the existing structure, the Design and Access Statement is the place to explain why the units are shaped as they are. Browse all 66 report types for the rest of a conversion pack, or See PlanScope pricing for what a single draft costs.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Does the space standard apply to every planning application?
It is an optional technical standard, so it applies where the local planning authority has adopted it through a local plan policy supported by evidence. Most urban authorities have. It applies automatically to dwellings created under permitted development rights, where compliance is a condition of the right rather than a matter of local policy choice.
Is a loft room with sloping ceilings counted?
Partly. Floor area where the headroom is below 1.5 metres does not count towards gross internal area, so the usable portion of a room in the roof is measured from the point where the ceiling reaches that height. The exception is the area under a stair, which is counted. This is why converted roof spaces lose more area on paper than they appear to on a plan.
How is built-in storage treated?
Built-in storage counts within the overall gross internal area, and the standard sets a minimum built-in storage area on top of the general minimum. A one bedroom two person dwelling has a storage requirement of 1.5 m square, and any storage provided above that figure can count towards the general internal area requirement if it is at least 900 mm wide and accessible from a habitable room or hallway.
What is the minimum size of a double bedroom?
The standard sets 11.5 m square for a double or twin bedroom, with a minimum width of 2.75 metres for at least one double bedroom. Single bedrooms are 7.5 m square with a minimum width of 2.15 metres. A room that meets the area figure but is too narrow fails, which catches long thin rooms formed by subdividing a larger space.
Does a scheme fail if one unit is undersized?
The unit fails, and on a prior approval application that is normally fatal to that unit. On a full application the authority weighs the shortfall as a policy matter and may still grant permission where the scheme has other merits, though most officers treat the standard as a floor rather than a target. Redrawing to lose one unit is usually the faster route.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.