Measurement and areas at developed design: what precision to expect before production drawings

Before discussing precision, the different definitions of area used in construction must be distinguished. They are not interchangeable, and confusing regulated floor area with habitable area or gross area can lead to significant pricing errors.

ℹ Which area for an outline estimate? For new-build apartments the economist generally works in regulated floor area, the statutory basis, or in habitable area, which speaks more directly to the brief. For an office project they will use gross or net usable area. The reference area must always be stated in the estimate or the measure: a ratio of 1,630 €/m² of habitable area does not give the same result as 1,630 €/m² of floor area on the same building.

The precision of a measure is not a question of the economist's competence but of the quality and completeness of the drawings supplied. The more precise, dimensioned and coordinated between disciplines the drawings are, the more reliable the measure. That is a basic axiom of construction economics.

The developed-design line is the one that concerns us. With properly dimensioned floor plans and elevations, a competent economist can produce measures to within 5 to 10 % on the principal trades. That precision is enough to confirm the budget envelope and prepare the tender package, but not enough to serve as the contractual basis of the works contracts.

Some quantities can be taken directly off developed-design drawings with good precision:

• Floor areas: measured level by level from dimensioned floor plans. The precision depends on the quality of the drawing and on whether wall and partition thicknesses are shown.
• Lengths of façade: measured on the elevations or in plan, allowing for returns and corners. A frequent source of error if the elevations are not yet available.
• Ceiling heights: read off the sections, with a direct bearing on masonry and painting quantities.
• The number of openings: windows, glazed openings and French doors counted on the drawings. Without details, the economist assumes dimensions that will be refined at tender.

Other quantities cannot be taken off developed-design drawings and necessarily rest on documented assumptions:

• Foundations: their nature and sizing depend on the ground investigation, often unavailable at this stage. The economist adopts an ordinary foundation assumption and says so explicitly.
• Technical services — mechanical, plumbing, electrical: with no engineering layouts at developed design, the economist applies lump-sum ratios per dwelling or per square metre. These are calibrated on comparable projects but can depart 15 to 20 % from the reality according to technical complexity.
• External works: these depend on the site plan and the connections to public utilities, often undefined at this point. A lump-sum provision is adopted.

ℹ The effect of incomplete drawings A developed-design drawing without dimensions obliges the economist to measure graphically, with an imprecision that can reach 10 % according to the scale and the quality of the drafting. Elevations not yet drawn, or floor levels not yet settled, can make some measurement impossible. In that case the economist must say so explicitly and increase the contingency accordingly.

One of the most unsettling things for a client is finding that the tender quantities differ from those at developed design, sometimes significantly. That is not necessarily the sign of an error: more often it is the normal reflection of the design being refined.

• The structural drawings, available at detailed design, change the sections and lengths of the concrete elements.
• The definitive services layouts reveal longer runs than the lump-sum assumptions allowed for.
• Construction details — façade sections, junction details — bring to light work not visible on the earlier drawings.
• The brief evolves between the two stages: a level modified, a plant room added, a specification changed on one package.

• The economist worked on undimensioned or partial drawings without saying so.
• The assumptions were not documented, making the variance impossible to trace.
• The brief changed without the measure being updated.

The golden rule: any significant variance, above 10 %, between the developed-design measure and the tender documents should be the subject of an explanatory note from the economist. Without that traceability the client cannot tell a normal evolution of the design from an initial error.

• Supply dimensioned drawings at developed design, even if the construction details are not yet settled.
• Show the thickness of walls, partitions and linings on the floor plans.
• Flag areas of particular complexity: façade set-backs, complex roofs, split levels.
• Update the drawings issued to the economist at every significant change of brief.

• The assumptions adopted for every quantity taken by assumption: nature of foundations, services allowances, external works.
• The drawings used, with their reference dates.
• The items where an imprecision above the normal has been identified.

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Precision at developed design runs at 5 to 10 % on well-defined trades, and can be lower on the services packages and the foundations for want of structural drawings and engineering layouts. That precision is sufficient to confirm the budget envelope and prepare the tender package, provided the assumptions adopted are explicitly documented. The key to good coordination between the two stages is traceability: knowing exactly what basis each quantity was produced on allows the changes between stages to be understood and explained, without needless conflict.

Reference basis of the figures

The ratios per square metre and per item on this page are stated in January 2026 values. They were updated from January 2025 values, the basis adopted for the whole cocoon, using the BT01 all-trades index published monthly by INSEE on its 2010 base: 132.0 in January 2025, 134.7 in January 2026, a coefficient of 1.0205.

The global amounts quoted as examples, and any rents, charges or receipts expressed per year or per night, remain on their own basis: a building cost index does not apply to them.

To bring the ratios forward, apply the usual revision formula: updated value = page value × (BT01 for the month in question / 134.7). The BT01 measures contractors' input costs, labour at 44.9 % and materials at 36.5 %, not the prices actually tendered.