Factors by impact
| Factor | Nature | Amplitude |
|---|---|---|
| Level of finish | decided | very high |
| Site and siting | imposed | high and unpredictable |
| Energy requirements | partly imposed | medium to high |
| Complexity of form | decided at design | medium |
| Size of project | structural | slight and regular |
| Market conditions and programme | imposed | variable |
Level of finish comes first because its amplitude is widest and because it acts on many items simultaneously. It is also the only factor the client decides entirely.
What is decided
Three families follow from a client decision. They constitute the real room for manoeuvre in the budget.
The level of finish is the most powerful and most immediate lever. It acts on floor and wall finishes, kitchen, sanitary ware and joinery, with ratios of one to four between ranges on some items. It is covered in the article on the level of finish.
Complexity of form is decided at design stage. Setbacks, multiple levels, composite roofs and articulated volumes increase lengths of façade, roof and junction without increasing usable area. The same brief in a simple volume costs less than a fragmented one.
The performance level targeted beyond the regulatory minimum is likewise a choice, with an investment logic to be set against operational savings.
What is imposed
Three families impose themselves, and they produce the unpleasant surprises. They are observed, not negotiated.
The site. Ground conditions, slope, access, party walls and the presence of water determine items design cannot remove. It is the least predictable factor and is covered in the article on site and siting.
Regulatory requirements. The applicable energy minimum depends on the region and is not negotiable. Differences between regions are covered in the article on energy requirements.
Market conditions. Between estimate and tender, material and labour prices move. On a project whose consent takes several months, that gap is not negligible and justifies a revision clause.
Size always works in the same direction
One structural factor deserves isolating, because it is regular and therefore predictable. It is the effect of scale.
The price per m² falls slightly as area increases, because foundations and roof do not track area proportionally, unlike partitions and finishes. The effect is measured from the sketch stage and depends on no decision.
In collective housing this effect can reverse where crossing a regulatory threshold triggers new obligations.
How to rank on a real project
Three questions reveal where the budget is actually decided. They are asked before going into the detail of individual items.
How much does the site weigh in the uncertainty. On an unknown site, ground investigation drives the budget more than the choice of floor tiles.
How much do finish items weigh in the total. On a turnkey contract they represent a major share and therefore a real lever. At closed shell they are outside the scope.
How long will the project foreseeably take. The longer it runs, the more market conditions weigh, and the more the revision clause matters.
What this means for an estimate
Three rules.
Address imposed factors before decided ones. Sound decisions are only possible once what cannot be decided is known.
Price the site uncertainty rather than estimating it. Ground investigation costs less than a foundation surprise.
Do not confuse variation with overrun. A project costing more because the level of finish was raised has not overrun; it has changed.
The articles in this branch
The article on the level of finish details the items concerned and the real amplitude. It is the factor opening the widest price amplitude.
The article on site and siting covers ground, slope, access and party walls. Those constraints are established before the land is bought.
The article on energy requirements by region compares the three regional frameworks. The three regions impose neither the same thresholds nor the same timetable.
The article on comparing two quotes gives the method for neutralising these factors. The method consists in bringing both quotes to the same scope.
This article reflects professional practice at the date of checking and serves as orientation. It does not constitute legal advice and does not replace assessment of the individual case.