How do I work out the real cost per square metre of a build?

Short answer

Fix your reference area and use it consistently — usually the total built floor area.
Add up actual costs across the four cost centres: labour, materials, subcontractors, equipment.
Add site overheads: site setup, utilities, transport, fees and permits.
Divide total cost by the reference area — the result is the real cost, not the quoted one.
Compare it with the original estimate, cost centre by cost centre, to see where the gap comes from.

Government Decision 907/2016 on the stages of technical and economic documentation — for publicly funded projects the general cost-estimate structure is the reference framework, and many private firms use it as their cost-breakdown model.
Accounting Law 82/1991 — every cost must be supported by a document: invoice, work statement, material issue note, payroll sheet.
Fiscal Code, Art. 281(7) — the moment the work counts as performed, on issuing or accepting the work statement, also sets the period the cost falls into.
Note: cost per square metre has no single legal definition. Without a stated reference area, two figures are not comparable.

Example: a house with 160 m² of total built floor area runs up actual costs of 512,000 RON — labour 148,000, materials 226,000, subcontractors 108,000, equipment 30,000. The real cost is 3,200 RON/m² against 2,950 RON/m² estimated, and almost all of the 250 RON/m² gap sits in materials.
Example: on an 800 m² warehouse, dividing cost by usable area (760 m²) instead of total built floor area inflates cost per square metre by over 5%. That is why the basis is always stated.

Comparing costs per square metre that were calculated on different reference areas.
Leaving out site overheads — setup, utilities, transport — and producing a cost that never happened.
Calculating the cost only once, at the end; until then there is nothing to correct.
Rolling client-requested extra works into the figure without separating them from the original budget.

Costs accumulate in real time across the four cost centres, from confirmed work statements and expenses.
The reference area is set on the project, so cost per square metre is recalculated with every new cost.
Budget vs. actual per cost centre shows exactly where the variance comes from.
The history of closed projects becomes the pricing baseline for the next similar job.