The structure of a construction budget
Before estimating a project's cost, the components of the overall budget must be distinguished. A frequent confusion equates the construction cost with the works cost alone, when the latter is only one part, admittedly the largest, of the total cost of an operation.
The all-in cost
The all-in cost is the most complete measure. It gathers four families of expenditure:
| The works cost | Structure, finishing trades, external works, technical equipment. Generally 60 to 70 % of the total according to the type of operation. |
|---|---|
| Design fees | Architect, structural, services and thermal engineers, economist, site coordinator, technical controller, health and safety coordinator. Between 8 and 14 % of the works cost according to complexity. |
| Land and administrative costs | Acquiring the site, legal fees, development taxes, contributions to roads and services. |
| Finance costs and insurance | Interest during construction, the completion guarantee, structural warranty insurance, marketing costs. |
Breaking down the works cost
- Structure: foundations, frame, masonry, roof structure, waterproofing, external joinery. 35 to 50 % of the works.
- Finishing trades: partitions, finishes, internal joinery, sanitary and heating equipment, electrics, plumbing. 35 to 45 % of the works.
- External works and surroundings: roads, services, landscaping, parking. 5 to 15 % according to the site and its complexity.
To those three are added contingencies: 3 to 5 % at advanced design, 8 to 10 % at sketch stage.
Estimating methods by design stage
Cost estimating sharpens progressively as the design advances. In France five levels of estimate are classically distinguished, corresponding to the stages of the design appointment.
Estimating by ratios, at concept stage, precision ± 20 to 30 %
With no detailed drawings, the estimate rests on cost ratios per square metre of floor area drawn from historic databases and site references, brought up to date through the BT01 index. It suffices to confirm the economic feasibility of an operation, but is too imprecise to commit contractually.
Estimating by functional elements, at outline design, precision ± 10 to 15 %
At outline design the project is broken into functional elements, foundations, structure, envelope, technical equipment, and priced from finer ratios. It is at this stage that overrun risks can be detected and the first corrective measures taken.
Quantitative estimating, at developed design and tender, precision ± 3 to 8 %
At developed design and tender, quantities are calculated from the drawings and specifications. The construction economist carries out the detailed measure and produces the pricing schedule or the bill of quantities. This is the reference estimate for the tender.
The factors that move costs
| Location | The Paris region shows premiums of 15 to 30 % over the national average. Major cities: 5 to 15 %. |
|---|---|
| Type and use of the building | Apartments, public buildings, certified offices, industrial infrastructure: the technical requirements and cost levels vary threefold. |
| Level of specification | Economical against high-end finishes can move the finishing trades by 30 to 80 %. |
| Site constraints | Difficult ground, party walls, a protected site, restricted access: each constraint generates its own premium. |
| Market conditions | Pressure on materials and labour can move prices by 5 to 20 % between two tenders 12 months apart. |
| Regulation | The environmental regulations, accessibility, seismic zones: regulatory obligations bear directly and increasingly on cost. |
ℹ The BT01 index is the barometer of building costs in France, published by INSEE. It measures the monthly movement of construction costs and is used to bring estimates forward and to revise prices in current contracts. Any budget more than 12 months old must be updated before it is used as a reference.
Levers for optimising the budget
Act on the structural approach
The choice of construction system, cast in-situ concrete, precast, timber frame, steel, bears directly on the structural cost and on the construction programme. Prefabrication can shorten the site by 20 to 40 %, reducing finance costs by as much.
Optimise the architectural form before the finishes
The design decisions with the greatest effect on cost are taken at sketch stage: the form of the building, the compactness of the plan, the number of levels, the ratio of façade area to floor area. Those decisions are near-irreversible once developed design is approved.
Rationalise the technical packages
Mechanical services, electrics and plumbing often represent 20 to 30 % of the works cost in new buildings. Simple, standardised solutions offer substantial optimisation without affecting perceived quality.
Optimise the calendar and group the packages
A tender launched in autumn or at the start of the year statistically produces more competitive offers. Grouping compatible packages can attract larger contractors with economies of scale.
Financial monitoring during construction
| The expenditure statement | Monitoring all committed expenditure, approved valuations and price revisions. Updated at every site meeting. |
|---|---|
| The financial report | A monthly summary of the operation's financial position, including the forecast out-turn cost. |
| Managing variations | Any departure from the original contract must be the subject of a variation signed before execution. Uncontrolled accumulation of variations is the principal cause of overrun. |
| The final account | The document that definitively settles the position between client and each contractor at the end of the contract. |
To go further: How to estimate the cost of a project · Cost per square metre by building type · Expenditure items on a construction site
Back to the main guide: Construction economics
Cost estimates, pricing schedules, bills of quantities, development appraisals, summary notes. Produce in minutes the deliverables you prepare manually today. → quostra.com · First project free
ℹ The nature of the figures The orders of magnitude on this page are expressed as percentages of a total, not as amounts. They therefore do not drift with the building cost index and need no updating: it is the distribution that is the reference, not the level. The amounts those percentages apply to must be taken at the date of the study. To bring them forward, use the BT01 all-trades index published monthly by INSEE on its 2010 base, which stood at 134.7 in January 2026.
The nature of the figures quoted
The percentages and amounts quoted on this page are orders of magnitude for framing, not measurements. They illustrate mechanisms and proportions, and they substitute neither for a tender exercise nor for an estimate prepared on drawings.
The ratios and orders of magnitude on this page are indicative for France. They must be adapted to each project according to its location, its brief and market conditions at the moment of tender.
Sources: the BT construction cost indices published monthly by INSEE on its 2010 base, used to update the figures · construction cost statistics from SDES, the statistical service of the ministry responsible for construction · regulatory texts published in the Journal officiel and consolidated on Légifrance for the requirements cited.