Measurements under permitted development

It is important to get measurements correct, especially when converting or building under permitted development rights. This might include height or floor space for example.

Individual classes within the GPDO can use their own measurement rules. For example, a new dormer under Class B is tested against the specific roof-enlargement provisions rather than a generic measurement rule. Always check the wording of the class being relied upon rather than assuming one method applies throughout the GPDO.

Floor space measurements

Article 2 defines “floor space” as the total floor space in a building or buildings, but it does not set out a universal measurement method. The important word is in: appeal decisions have repeatedly treated GPDO floor space as being measured within the external walls rather than by taking the external footprint of the building.

That is different from the method used for some other planning calculations. For example, the Fees for Applications Regulations use their own measurement rules for their own purpose. Those rules do not automatically supply the meaning of floor space in the GPDO.

In appeal APP/R5510/W/19/3229135, concerning the former Class PA, the Inspector rejected the council’s attempt to measure gross floor space from the outside of the external walls. He treated “gross” as emphasising a total without deductions, so areas such as staircases remained within the calculation. Although Class PA is no longer a current permitted development route, the decision is useful when interpreting the GPDO terminology.

The same broad approach has since appeared in Class Q decisions. Inspectors have measured to the internal face of the perimeter walls and included internal walls within the floor-space calculation. The present Class Q limits therefore need to be tested using the actual enclosed floor space rather than the external footprint, while taking care not to invent deductions for partitions, stairs or other areas which the GPDO does not exclude.

There is an important caution: “inside the external walls” is not necessarily the same as “habitable room area”. Covered or partly open areas can sometimes still be floor space within a building, depending on the structure and the wording of the particular class. Where a numerical limit is tight, measure conservatively and check the exact GPDO provision being relied upon.

Measurements for heights

Height might apply to a fence or an outbuilding for example, or it might be used in connection to the eaves of a house.

For outbuildings, references to height are measurements taken from ground level. Note, ground level is the surface of the ground immediately adjacent to the building in question and would not include any addition laid on top of the ground such as decking. Where ground level is not uniform (for example if the ground is sloping), then the ground level is the highest part of the surface of the ground next to the building.

Article 2 defines height as  ‘Unless the context otherwise requires, any reference in this Order to the height of a building or of plant or machinery is to be construed as a reference to its height when measured from ground level; and for the purposes of this paragraph “ground level” means the level of the surface of the ground immediately adjacent to the building or plant or machinery in question or, where the level of the surface of the ground on which it is situated or is to be situated is not uniform, the level of the highest part of the surface of the ground adjacent to it.’

This is important when measuring the height of buildings and eaves, but the individual permitted development class may add its own limits or measurement rules. If existing works complied with the GPDO when they were carried out, they were permitted development at that time and their lawfulness can later be confirmed by a certificate where appropriate. If the works did not comply, the GPDO cannot simply be applied retrospectively to cure the original breach; a retrospective planning application, enforcement route or other lawful solution may instead be required.

Fences, gates and walls need particular care because they fall under Part 2 Class A and are expressly excluded from the GPDO definition of a “building”. The Article 2 rule which measures a building from the highest adjacent ground level therefore should not simply be copied across to a fence.

Part 2 Class A allows a maximum of 1 metre above ground level where a gate, fence, wall or other means of enclosure is erected or constructed adjacent to a highway used by vehicular traffic, and 2 metres in other cases, subject to the rest of the class. The GPDO does not provide an equivalent universal rule saying that the highest ground on the owner’s side must always be used. Where levels differ, the context matters. In appeal APP/D0121/C/20/3246774, for example, the Inspector treated the highway as the reference point for fencing adjacent to that highway rather than the higher garden level within the site. Do not assume that raising ground levels or measuring only from the higher side will bring an otherwise oversized boundary treatment within permitted development.

Measurements for depth

For an extension this is measured from the original rear wall of the house. The technical guidance says ‘Measurement of the extension beyond the rear wall should be made from the base of the rear wall of the original house to the outer edge of the wall of the extension (not including any guttering or barge boards).’

If you join or build alongside an existing extension, measure the resulting enlargement against the Class A limits that apply to the whole proposal. A rear enlargement does not simply become a “side extension” because it touches or extends beyond an original side wall. Depending on the geometry, the proposal can engage both the rear-extension and side-extension limitations, including the separate width and height controls. See our Class A extensions guide for the detailed tests.

if you join an outbuilding or shed for example, that outbuilding becomes part of the extension and the combined area must meet the depth limits. See this page for more on extensions.

What to check for under permitted development regarding measurements

If something is not defined in Article 2 then it may have in an interpretation elsewhere in the GPDO. This might be within the class itself or within a separate paragraph, such as Paragraph X of Part 3.

Be aware that because something isn’t restricted now, that does not mean that it will not be in future. The legislation within Planning Geek should always be up to date.

Occasionally case law might come into play, but as with floor space, if it is not defined within the GPDO then we need to work out exemptions that are defined and work back from that.

Measurements Page Updated: 2nd September 2026

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