How to calculate the internal GIA of a property?

The Technical Planning Guidance States:

The Gross Internal Area (GIA) of a dwelling is the total floor space measured between the internal faces of perimeter walls. This measurement includes partitions, structural elements, cupboards, ducts, flights of stairs, and voids above stairs. It is denoted in square meters (m²).

According to the standard:

• The dwelling must provide at least the gross internal floor area and built-in storage area as set out in Table 1 below.
• A dwelling with two or more bedspaces must have at least one double (or twin) bedroom.
• For one bedspace, a single bedroom must have a floor area of at least 7.5m² and a width of at least 2.15m.
• To provide two bedspaces, a double (or twin) bedroom must have a floor area of at least 11.5m².
• One double (or twin) bedroom should be at least 2.75m wide, and every other double (or twin) bedroom must be at least 2.55m wide.
• Any area with a headroom less than 1.5m is not included in the GIA unless it is used solely for storage. For storage under stairs, assume a general floor area of 1m² within the GIA.
• Any other area used solely for storage with a headroom of 900-1500mm (such as under eaves) counts at 50% of its floor area. Areas lower than 900mm are not counted.
• A built-in wardrobe contributes to the GIA and bedroom floor area requirements. However, it should not reduce the room’s effective width below the minimum specified. Any built-in area exceeding 0.72m² in a double bedroom and 0.36m² in a single bedroom counts toward the built-in storage requirement.
• The minimum floor-to-ceiling height is 2.3m for at least 75% of the GIA.

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