How are the ratios used in an outline estimate calculated?

A construction cost ratio is a reference unit cost expressing the price of an element, or a set of elements, against a meaningful unit of measure — usually the square metre of floor area, habitable area or footprint. It condenses into a single value all the construction costs associated with a type of building under given conditions.

A ratio is not a universal rule. It is a central value extracted from a sample of comparable projects, accompanied by a range of variation reflecting the dispersion of the costs observed. It is that range, not the central value alone, that constitutes the useful information for an outline estimate.

ℹ Ratio or unit rate: what is the difference? A construction ratio — say 1,680 €/m² of floor area for standard apartments — is an aggregate covering all the trades of a project. A unit rate — say 95 €/m² of blockwork to elevations — is the cost of one specific item, used in the developed-design measure. The outline estimate works with global ratios. The developed-design measure works with unit rates item by item. Moving from one to the other is one of the great steps in refining a project economically.

The ratios construction economists use fall into several families, according to the level of aggregation sought and the nature of the works to be estimated.

Family of ratio	Unit	Principal use	Examples
Global all-in ratio	€ excl. VAT / m² floor area	Global outline estimate	1,680 €/m² for standard apartments in the Paris region
Ratio by trade	€ excl. VAT / m² habitable	Calibrating a package	Structure: 560 €/m², heating and ventilation: 290 €/m²
Functional ratio	€ excl. VAT / functional unit	Specific programmes	Care-home room: 85,000 – 110,000 € incl. VAT
External works ratio	€ excl. VAT / m² of plot or linear m of road	Surroundings and services	Ordinary external works: 45 – 80 €/m² of site
Technical package ratio	€ excl. VAT / m² treated	Services packages	Electrics, power and data: 120 – 180 €/m² habitable
Regional cost index	Multiplier	Geographic correction	Paris region: × 1.15; average provincial: × 1.00

In practice an outline estimate combines several kinds of ratio: a global one to obtain a first band, then ratios by trade or by technical package to refine the items carrying the most weight or the most uncertainty. This hybrid approach is more precise than a single global ratio while remaining compatible with the level of information available at this stage.

The raw material of a ratio is feedback from completed sites. Every finished project is a source of data: the final contract sums after variations, the real areas, the level of specification, the geographic context, the date of completion. A practice builds an internal database progressively by capitalising on the projects it has delivered.

That base is enriched by external sources: publications of the profession's national union, regional construction cost observatories, and commercial databases. These supply data aggregated nationally or regionally, useful for calibrating ratios on building types poorly represented in the practice's own portfolio.

Not all data collected is directly comparable. Before a project enters the ratio base, the economist normalises it: converting areas to a common definition, excluding atypical items such as special equipment, furniture and artwork, and stripping out exceptional variations linked to damage or unforeseen discoveries.

They also filter out projects too far from the median — the outliers — so that one exceptional project does not distort the reference. The statistical robustness of a ratio depends on the size and quality of the underlying sample.

Construction costs move over time under the effect of material inflation, regulatory change — new thermal, accessibility and seismic requirements — and pressure on labour in the construction market. A ratio built on projects delivered five years ago can be significantly under-valued if it is not updated.

Updating is done by applying the BT indices published monthly by INSEE: BT01 for all trades, BT02 for earthworks, and a specific index for particular packages. These bring a historic cost forward to the anticipated tender date.

ℹ A worked example of updating A project delivered in January 2020 cost 1,580 €/m² of floor area. The BT01 index stood at 111.8 in January 2020 and 134.7 in January 2026. The updated ratio is therefore 1,580 × (134.7 / 111.8) = 1,904 €/m². This updating is indispensable in any outline estimate produced from references more than 18 months old.

The table below gives indicative ratios for the principal project types in France, for a tender in 2025 and a provincial location outside the Paris region and areas attracting site allowances. They are market references and vary with the specifics of the project and the site.

Type	Level of specification	Indicative ratio (€ excl. VAT/m² floor area)	Principal correction factors
House	Standard	1,400 – 1,700	Basement, complex roof, sloping site
House	High-end	1,900 – 2,500	Fine materials, swimming pool, home automation
New apartments	Standard, typical off-plan sale	1,600 – 1,900	Building height, number of lifts, parking
New apartments	Serviced residence or high-end	2,200 – 2,800	Communal facilities, premium finishes
New offices	Standard	1,700 – 2,100	Floor-to-floor height, services, façade quality
New offices	High performance, certified	2,300 – 2,900	Certifications, double-skin façades, building management
Apartment refurbishment	Partial renovation	700 – 1,100	Condition of the existing fabric, asbestos or lead, occupied premises
Apartment refurbishment	Heavy renovation	1,200 – 1,800	Complete overhaul, discoveries on site
Public buildings, schools and sports facilities	Standard public brief	2,000 – 2,600	Regulatory requirements, accessibility, acoustics

Note: these ratios cover construction work excluding fees, design studies, utility connections and VAT. For the Paris region, apply an uplift of 10 to 20 % according to location. For the French overseas territories, logistical premiums can reach 30 to 50 %.

Even well calibrated, a ratio is an average value smoothing very diverse realities. It does not automatically capture certain specific costs that can weigh heavily on a budget:

• Special foundations — micropiles, bored piles, a reinforced raft — arising from unfavourable ground: 10 to 25 % on top of the structural cost.
• Site constraints in dense urban settings: restricted access, no tower crane, no storage on site.
• Hazardous materials on refurbishment: asbestos and lead removal, whose cost varies widely with the nature and quantity involved.
• Environmental certification requirements: 5 to 15 % extra according to the level sought.
• Specific equipment tied to the brief: goods lifts, cold rooms, laboratories, clean rooms.

This is why an outline estimate is not a simple multiplication of area by ratio. The economist analyses the project, identifies the specifics that depart from the assumptions behind the reference ratio, and applies the necessary corrections. That analytical step is irreplaceable, and it is what distinguishes an estimate produced by a professional from a figure generated by an automatic calculator.

ℹ The trap of online tools Many online tools offer an automatic estimate based on area and building type. They give a useful order of magnitude for a first approach, but cannot replace an economist's analysis: they take no account of ground conditions, site constraints, the state of the existing fabric on refurbishment, or the real level of specification. An outline estimate produced by an economist incorporates all of these — that is where its value lies.

Quostra maintains a database of ratios fed by the projects handled on the platform, updated quarterly against the BT indices and enriched by regular benchmarking of market data. It covers the principal types of new build and refurbishment, broken down by region and by level of specification.

For every outline estimate produced, the Quostra economist documents explicitly the ratios used, the reference sources and the correction factors applied. That traceability lets the client and the architect understand and challenge the assumptions adopted.

You have better things to do. Submit your project on quostra.com and an economist produces your outline estimate from calibrated, up-to-date ratios.

Construction cost ratios are powerful tools, provided you understand how they are built, updated and applied. Derived from real site data, brought forward through the BT indices and corrected for the specifics of the project, they let the economist produce a reliable outline estimate from the first sketches. Their limit is precisely what justifies bringing in a professional: no ratio replaces analysis of the particular context of each project.

Reference basis of the figures

The bands on this page are stated in January 2026 values. They were updated from January 2025 values 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.

To bring them 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. The two series diverge as soon as margins move: over 2024 the BT01 rose by 0.9 % while the construction cost observed by the SDES survey rose by 2.1 %. These bands remain orders of magnitude, to be set against an estimate specific to the project.