1. Why architectural choices weigh so heavily
A building is a complex system where every design decision has cascading repercussions on other packages and other items. Those are the technical interfaces, and they are precisely where the premiums hardest to anticipate without economic expertise hide.
Take a simple example: raising the floor-to-ceiling height from 2.70 m to 3.00 m in a five-storey office building. The decision looks minor, 30 centimetres. Yet it generates cascading premiums: the total height rises by 1.50 m, which affects the façade at + 5 %, the stairs at + 3 %, the lift shaft at + 2 %, the volume to be cooled at + 6 %, and the heating cost at + 4 % over the life of the building. On a 2,500 m² building at 2,000 €/m² of usable area, that single choice represents 120,000 to 200,000 € of premium.
ℹ The rule of cascading effects In construction no technical decision is isolated. Every choice, of structure, of façade, of geometry, of height, triggers a series of effects on other items. An economist evaluating a technical choice does not simply price the package directly concerned: they reconstruct the whole cascade. That is why bringing the economist in at outline stage, while the structuring choices are still reversible, is infinitely more valuable than at developed design, where they can only record the consequences.
2. The choices with the greatest economic effect
The table sets out the most frequent technical decisions with a strong budget effect, organised by family. For each, the standard option serves as the reference and the premium or complex option is quantified as a relative premium.
| Technical choice | Standard option | Premium or complex option | Premium | Effect on other packages |
|---|---|---|---|---|
| Façade, standard cladding | Painted fibre cement | Lacquered aluminium composite | + 30 – 60 % | No effect |
| Façade, standard external insulation | Polystyrene with render | Mineral wool with timber cladding | + 40 – 80 % | Structural premium above 40 kg/m² |
| Façade, curtain walling | Masonry wall with windows | Aluminium curtain walling with glazing | + 150 – 300 % | Premium on weathering and joints |
| Structure, standard spans below 8 m | Ordinary concrete frame | Long-span steel frame | + 20 – 40 % | Foundations, if loads increase |
| Structure, cantilever beyond 3 m | Traditional structure | Post-tensioned concrete cantilever | + 60 – 150 % | Strengthened foundations mandatory |
| Structure, standard flat roof | Concrete slab with waterproofing | Accessible roof garden | + 80 – 150 % | Structural premium plus permanent load |
| Basement, one level at −3 m | General raft | Tanking where the water table is high | + 40 – 100 % | Permanent pumping where the table is active |
| Basement, two levels at −6 m | Soldier pile wall | Diaphragm wall with reinforced concrete | + 100 – 200 % | Premium on external works and neighbours' foundations |
| Geometry, simple rectangular plan | The reference | , | 0 | , |
| Geometry, non-right angles | Rectangular plan | Trapezoidal or polygonal plan | + 10 – 25 % | Bespoke joinery plus adapted finishing trades |
| Geometry, floor-to-ceiling height | 2.50 m in housing | 3.00 m or more | + 5 – 8 % per 10 cm | Façade, heating and volume |
| Roof, simple form | Standard two-pitch roof | Complex multi-pitch roof | + 30 – 80 % | Roof structure, flashings, gutters |
| Roof, zinc or slate | Standard concrete tile | Standing-seam zinc or slate | + 80 – 200 % | Adapted roof structure |
Three families concentrate most of the economic surprises: the basement, whose cost is systematically under-estimated, especially where the water table is high; the façade, where changing system can triple the cost of the package; and geometric complexity, whose cumulative effect across every package, bespoke joinery, complex weathering, adapted finishing trades, is often greater than its isolated effect on the structure.
3. The matrix of impact against reversibility
The other critical dimension of technical choices is reversibility: up to which stage can a decision be changed without prohibitive cost? The matrix crosses economic impact with the window of reversibility for the most structuring decisions.
| Technical decision | Cost impact | Reversibility | Limiting stage | Recommendation |
|---|---|---|---|---|
| Number of storeys above ground | Very high | Nil after permission | Concept, permission | Decide before permission; it affects foundations, façade and lift |
| Presence of a basement | Very high | Nil after permission | Concept, permission | Model with and without a basement from feasibility |
| Structural system: concrete, steel, timber | High | Low after outline | Outline, developed | Price two or three variants at outline before deciding |
| Type of façade | High | Low after developed | Developed design | Compare cladding, external insulation and curtain walling ratios |
| Floor-to-ceiling height | Moderate | Nil after permission | Concept | Each additional 10 cm adds 5 to 8 % to the building |
| Roof complexity | Moderate to high | Low after outline | Outline design | Compare simple against complex roof at outline |
| Internal finishes | Moderate | High until detailed design | Detailed design | Adjustable to budget; the regulating lever at the end of design |
| Mechanical systems | Moderate | Moderate at developed design | Developed design | Model heat pump against boiler against district heating |
The matrix identifies clearly the decisions that must be taken at concept and outline stage, before the planning application: the number of storeys, the presence of a basement, the floor-to-ceiling height. Those choices are irreversible once the application is filed. Deciding them without a comparative cost estimate is a major economic risk.
ℹ The basement: the most under-estimated choice Adding a basement level is the technical decision with the least favourable ratio of premium to perceived benefit in current French construction. A level of underground parking costs 28,000 to 40,000 € a space, that is three to five times the cost of a surface or deck space. Where the local plan imposes a parking ratio, the question to ask before choosing a basement is whether a lawfully acceptable alternative exists, sharing, a relaxation, surface parking. In most cases the basement is chosen by default, not by necessity.
4. Economical alternatives that preserve the essence
The purpose of economic analysis of technical choices is not to suppress architectural ambition but to optimise it. For every budget constraint there is generally one or more alternatives that preserve the essence of the intention at a significantly reduced cost.
| Budget constraint | The expensive option to avoid | The economical alternative | Saving | What is not sacrificed |
|---|---|---|---|---|
| Façade with a limited budget | Fully glazed aluminium curtain walling | Composite cladding with insulated windows | − 40 to 60 % | Thermal performance maintained |
| Basement where parking is needed | Two basement levels at −6 m | One level plus surface compensation | − 40 to 60 % | The number of spaces, subject to the local plan |
| Structure with long spans | Post-tensioned concrete | A precisely calculated steel frame | − 10 to 20 % | Identical structural performance |
| Roof with architectural interest | Standing-seam zinc on a complex roof | Zinc on a simple two-pitch roof | − 30 to 50 % | The fine material retained |
| Clear height for atmosphere | 3.20 m throughout | 3.20 m at ground level, 2.70 m above | − 5 to 8 % | The sense of volume where it is seen |
| Finishes with strong visual impact | Timber flooring on every level | Timber in the entrance and living space, tiling elsewhere | − 20 to 35 % | The fine finish where it shows |
| Maximum comfort in services | Underfloor heating, heat pump and heat-recovery ventilation | Heat pump with radiators and simple extract | − 15 to 25 % | Thermal comfort maintained |
The most fertile trade-off is often to concentrate the fine materials and premium specification where they have the most perceived impact, the entrance common areas, the street façades, the principal living spaces, and to take economical solutions in the less visible parts. That logic of a budget for the stage is the logic of well-run major architectural projects.
5. Timber and cross-laminated structures: advantage or premium?
Cross-laminated timber and timber structures are developing rapidly in French construction, driven by the environmental regulations and decarbonisation objectives. Their economic position is often misunderstood.
When timber is economically advantageous
Timber is advantageous in three configurations: extending a building upward, where its lightness reduces the strengthening required to the existing structure; modular offsite construction, where prefabrication shortens the site; and programmes with an imperative delivery date, where the speed of erection can save two to four months on a repetitive scheme.
When timber generates a premium
Timber generates a premium in two configurations: tall buildings, from six or seven storeys, where fire requirements impose expensive protection; and high seismic zones, where timber connections are more complex than concrete.
The order of magnitude of the timber premium against concrete on a standard programme is 5 to 15 % on the structural cost, partly offset by the speed of erection through reduced carry and site set-up costs.
6. How the economist supports technical choices
The economist's intervention on technical choices takes three concrete forms during design.
- The study of variants at outline stage: for each structuring choice, type of façade, structural system, presence of a basement, the economist produces a comparative pricing of the two or three options in play. That document lets client and architect take an informed decision rather than a bet.
- Modelling the cascade: for decisions on height, geometry or roof complexity, the economist models the effect across every package concerned, not only the principal one. That modelling often reveals that the real economic impact is two to three times the apparent one.
- Judging by value: faced with a budget constraint on an ambitious project, the economist can rank the items by their ratio of perceived value to real cost. That ranking lets architect and client identify where it is rational to concentrate the budget, and where an economical alternative sacrifices nothing essential.
In summary
The structuring architectural choices, basement, façade, height, geometry, roof, can generate premiums of 20 to 300 % according to the option taken. The window of reversibility is narrow: decisions on the number of storeys, the presence of a basement and the floor-to-ceiling height must be taken before the planning application, with a comparative estimate in hand. The basement is the most systematically under-estimated choice, at 28,000 to 40,000 € a space. Economical alternatives reduce the cost of an item by 20 to 60 % without sacrificing the essence of the architectural intention, by concentrating the quality budget where it has the most perceived impact. The study of variants at outline stage, produced by the economist, is what turns these risks into informed decisions.
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.
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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.