How to calculate cost per m2: floor areas and conventions

Floor area	What it includes	Effect on cost per m2
Gross floor area	Everything within the external perimeter, walls included	The largest: lowest cost per m2
Saleable area	Net area plus weighted shares of walls, balconies and common parts	Intermediate
Net usable area	Only the internal walkable surface, walls excluded	The smallest: highest cost per m2

The effect is quantifiable: between gross and net area the typical gap is 20-30 per cent, so the same spending produces an appreciably different cost per m2 depending on the denominator used. An example closes the matter.

Spending of EUR 216,000 on a house with 140 m2 gross, 128 m2 saleable and 112 m2 net. The same amount produces three different costs per m2.

Area used	Calculation	Cost per m2
Gross (140 m2)	216,000 / 140	EUR 1,543
Saleable (128 m2)	216,000 / 128	EUR 1,688
Net (112 m2)	216,000 / 112	EUR 1,929

Three figures, one building, one amount of spending. Anyone comparing EUR 1,543 and 1,929 believing they are comparing two quotes is comparing two denominators.

To add: ancillary areas (basements, lofts, balconies, porticoes) follow their own weighting rules (typically 25, 50 or 60 per cent depending on use and the rule applied). Including them at 100 per cent artificially lowers the cost per m2 and is one of the most widespread unintentional tricks in commercial communication.

By professional convention, the construction cost covers the building and services works needed to build, and excludes. The exclusions must always be stated alongside the figure.

Excluded item	Order of magnitude
Land	20 - 30% of the overall budget
Urbanisation fees and building contribution	per municipality and use
Professional fees (design, supervision, safety, testing)	10 - 15% of the works
Utility connections	EUR 4,000 - 10,000 in residential
Furniture and equipment	variable, excluded by definition
VAT	per applicable rate
Contingencies	8 - 15% depending on new build or existing
Finance costs	per the operation's structure

The sum of these items is typically worth 25-30 per cent more than the construction cost, before land. That is the systematic gap between the figure in circulation and the budget actually needed, and the reason every article in this cluster works from construction cost through to the operation's total.

Comparing different floor areas. The first and most frequent: two per-m2 figures are not comparable unless the reference area is stated.

Comparing different scopes. Turnkey against shell, fit-out against shell and core, a structural warehouse against a finished one: this cluster's bands repeat it for every building type.

Comparing different dates. Price lists move with materials and energy prices: a reference from three years ago describes another market. Every price per m2 has a date, and it must be read.

Comparing different territories. The gap between Italian macro-areas reaches 25-35 per cent, as developed in the article on north-south cost differences.

To use a cost per m2 found or received, four steps in order. One: establish the reference floor area and recalculate if necessary. Two: verify the scope of what the cost includes, and add the excluded items to reach the operation's budget. Three: correct for territory, context and site with the factor coefficients. Four: check the plausibility of the composition with the trade shares in the article on the construction cost breakdown.

And a method reminder: cost per m2 is a tool for first approximation and for checking, never for estimating. A reliable estimate is built by measuring the project's quantities and applying verified unit prices to them; cost per m2 serves to tell, at a glance, whether that estimate is plausible.