1. Introduction
Gross Internal Area (GIA) and Net Internal Area (NIA) are the two most common floor area measurements in UK property. They give everyone involved a consistent way to describe the size of a building, and they are used for planning, design, leasing, valuation and building surveys.
This guide explains the difference between GIA and NIA, how each is measured and when each is typically used. Both are defined in the RICS Code of Measuring Practice.
2. What is GIA?
Gross Internal Area is the whole area inside a building, measured to the internal face of the external walls on each floor. It includes everything inside that line: circulation, stairwells and lift shafts, toilets, plant rooms, internal walls and columns.
3. What is NIA?
Net Internal Area is the usable area within a building, also measured to the internal face of the perimeter walls. It leaves out the areas that cannot be occupied, such as common stairwells and lift shafts, toilets, plant rooms, permanent corridors and the space taken up by columns and internal structural walls.
4. Key differences
| Aspect | GIA | NIA |
|---|---|---|
| Definition | Total internal floor area | Usable internal floor area |
| Measured to | Internal face of external walls | Internal face of perimeter walls |
| Includes | All internal spaces, including circulation, toilets, plant and structure | Only space that can be occupied and used |
| Common uses | Planning, CIL, building costs, industrial valuation | Office and retail leasing, valuation, space planning |
5. Common applications
GIA and NIA are used across a wide range of property and construction work, including planning applications, building design, leasing, valuation, business rates, workspace planning and refurbishment. Accurate measurement gives architects, developers, property professionals and building owners figures they can rely on.
6. FAQs
Which measurement should I use for planning applications?
Planning authorities usually ask for Gross Internal Area. GIA is also the basis for calculating the Community Infrastructure Levy (CIL). Check the validation checklist of your local planning authority, as requirements can vary.
Can GIA and NIA be measured for existing buildings?
Yes. Both are routinely measured on site for existing buildings, usually as part of a measured building survey. The survey records the internal faces of walls so the areas can be calculated accurately.
Do GIA and NIA include external walls?
No. Both are measured to internal wall faces. If you need the area including external walls, that is Gross External Area (GEA), a separate measurement.
Why are GIA and NIA important for building surveys?
They are used for valuation, leasing, rent reviews and business rates. For example, offices and shops are usually valued on NIA, while industrial and warehouse buildings are usually valued on GIA. An error in the area feeds directly into rent, rates and price.
