Floor areas
Net internal area or gross internal area: which do you need?
By the Section Scale team · Published 7 November 2026
The difference
Gross internal area (GIA) is the whole floor area inside the external walls, measured at each floor. It includes internal walls, stairs, lift shafts and plant rooms.
Net internal area (NIA) is the usable space. It leaves out common parts and areas you cannot really use, so it is always smaller than the gross figure.
Gross external area (GEA) goes one step further than GIA and includes the thickness of the external walls.
What net internal area leaves out
- Toilets, stairwells and lift wells
- Plant rooms and other service areas
- Internal structural walls and columns
- Corridors and other areas shared with other occupiers
- Areas with headroom below 1.5 metres
Which one is used where
- Shops and offices: usually let, sold and valued on net internal area
- Industrial units and warehouses: usually gross internal area
- New homes and building cost estimates: usually gross internal area
- Planning and site coverage: often gross external area
Which measuring standard
Most UK agents and valuers use the definitions in the RICS Code of Measuring Practice. Some clients, particularly for offices, ask for the International Property Measurement Standards (IPMS) instead. If your lease, agent or valuer names a standard, tell us and we will measure to it.
Why measure on site
Floor areas copied from old particulars or architect drawings are often wrong, and rent is usually quoted per square foot. On a 2,000 square foot shop let at £30 per square foot, a 3% error is £1,800 a year, every year of the lease.
We measure every floor on site and show the areas on the floor plans, broken down by floor and unit, in both square metres and square feet.
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