Architect cost estimate in Switzerland: accuracy, tolerance and liability
- Simon Käslin
- Jul 1
- 8 min read
Updated: 6 hours ago
An architect cost estimate in Switzerland is not simply a non-binding gut feeling: depending on the project phase, it has a defined degree of accuracy and a permissible range, and the architect can be held to it to a certain extent.
As long as the project remains unchanged, the costs should stay within the tolerance. If the detailed cost estimate (Kostenvoranschlag) is significantly exceeded, the architect may be liable.
However, there are clear exceptions, such as special requests from the client or unforeseen events like boulders in the subsoil. Anyone who understands this logic knows what they can rely on and where there is leeway. raumwert plus helps clients place cost figures correctly and secure them contractually.
A cost estimate is a binding range, not a non-binding gut feeling.
Table of contents
How reliable is your cost estimate?
A brief overview shows what you can rely on and where there is leeway.
1. What is the architect cost estimate in Switzerland?
A cost estimate is the architect's professional assessment of what a construction project is likely to cost. It is based on the current planning stage and becomes more precise as the planning progresses. A cost figure is only ever as accurate as the basis on which it rests.
In an early phase with little information, it is rough; in a later phase with detailed plans, it is reliable.
The right expectations are crucial. A cost estimate is not a fixed price, but neither is it a non-binding gut feeling. It is a forecast with a defined range, for which the architect must calculate carefully.
2. What cost accuracy levels are there?
In Switzerland, a tiered system of accuracy is used, with levels increasing as the planning progresses. In simplified terms, there are three levels.
Level | Project phase | Tolerance (guideline value) |
Cost framework | Preliminary studies | approx. ± 25% |
Cost estimate | Preliminary project | approx. ± 15 to 20% |
Detailed cost estimate | Construction project | approx. ± 10% |
Guideline values according to SIA 102. For conversions and renovations, the tolerance is greater. Record the agreed level in the contract.
Cost framework: Very early, rough estimate in the preliminary studies, with a wide range.
Cost estimate: In the preliminary project, based on a developed concept, with a medium range.
Detailed cost estimate: In the construction project, based on detailed plans, with a narrow range.
These levels are not a detail but the heart of the matter: anyone assessing a figure must know which level it belongs to. A number without a stated degree of accuracy is of little value.
3. How accurate does a cost estimate need to be?
SIA 102 provides guidelines for accuracy, which become stricter with each phase. For orientation:
A cost estimate from the preliminary project phase may have a tolerance of around plus/minus twenty percent, while a detailed cost estimate from the more detailed construction project may only have a tolerance of around plus/minus ten percent. For conversions and renovations, the tolerance is greater because the existing structure carries more uncertainty.
The crucial point is that the tolerance is named and ideally agreed in the contract. A reputable architect will not say "that will cost 900,000 francs", but rather "at the current stage I expect 900,000 francs with an accuracy of plus/minus ten percent".
This transparency is not only good style, it is also legally relevant.
4. Can the architect be held liable for the costs?
Yes, up to a certain point. Herein lies the most important point, which many clients are unaware of: As long as the actual costs remain within the agreed tolerance, the client bears any additional costs themselves. Within this range, there is no breach of contract.
If the detailed cost estimate is exceeded by more than the agreed tolerance, a breach of contract by the architect is presumed. He cannot then simply argue that it was "only an estimate". That after-the-fact excuse does not work, because careful calculation is part of his duties. The legal basis is the duty of care of the mandatary under Art. 398 para. 2 of the Swiss Code of Obligations (CO), which can be read at fedlex.admin.ch), confirmed by the case law of the Federal Supreme Court (BGE 119 II 249).
5. What does liability mean in concrete terms?
Liability is conditional, not absolute. Three conditions are central. First, there must be fault, meaning careless calculation or supervision. Second, the client must have suffered a loss.
This so-called reliance loss is calculated as the difference between the actual construction costs and the subjective value the building has for the client. Third, the cost overrun must not be caused by an exception.
The concept of loss is the most sensitive point here. In return for the additional costs, the client often also receives added value, a more valuable building. The recoverable loss is therefore frequently smaller than the pure cost overrun. This is precisely why we speak of conditional liability.
It does apply, but it is subject to conditions and must be assessed case by case.
6. What exceptions are there?
Not every overrun leads to liability. There are two important exceptions that are not reflected in the estimate.
Order changes and special requests: If the client changes their requirements during the planning or construction phase, this alters the basis of the estimate. They bear these additional costs themselves.
Unforeseen circumstances: Circumstances that nobody could have known about are not covered by liability. The classic examples are boulders or poor ground conditions that appear during excavation. Such risks are not included in a normal cost estimate.
The architect can therefore be released from liability if he proves that the additional costs stem from one of these exceptions. The burden of proof lies with him.
7. How realistic is it to actually hold the architect liable?
An honest assessment is essential: In theory, the architect is liable, but in practice, enforcement is challenging. Three hurdles are crucial.
First, the client must prove fault, meaning careless calculation or supervision. Second, they must demonstrate an actual loss. This is precisely where many claims fail: the additional costs are often offset by added value, because the building is correspondingly more valuable.
This benefit in value is offset, so the recoverable loss may be small or even zero. Third, the client must show that with a correct estimate they would have decided differently, that is, not built at all or built smaller.
The practical consequence is clear: relying on later liability is risky. Prevention is more effective. A tolerance or cost ceiling agreed in the contract, a clean cost structure and an early independent review offer better protection than a dispute afterwards.
Liability is a safety net, not a substitute for careful agreements made at the outset.
8. What information obligations does the architect have?
A central duty is often underestimated: as soon as it becomes apparent that the costs will exceed the detailed cost estimate, the architect must inform the client immediately. Only then can the client react in time, examine savings measures or adjust their wishes.
If the architect informs the client too late, the client can no longer intervene on the construction site. This delayed or missing information is itself a breach of duty. Ongoing, transparent cost tracking is therefore part of the duty of care owed.
9. What is the difference between the degree of accuracy and the cost ceiling?
The degree of accuracy is the normal, phase-dependent tolerance of a cost figure. The cost ceiling is something different: it is an additional contractual agreement setting a maximum amount that the costs must not exceed.
Only then is a genuine cost guarantee created, and the architect must plan the project specifically towards this cost target.
A cost ceiling is advisable when the client needs maximum certainty, for example in the case of tight financing. However, it must be clearly defined: what scope of services is included, how order changes are handled and what happens in the event of unforeseen circumstances.
10. How can you recognise a reliable cost estimate?
Even as a layperson, you can assess quality if you pay attention to transparency and structure.
Is the degree of accuracy stated (for example plus/minus ten percent)?
Is the estimate structured (according to the Swiss construction cost classification BKP or eBKP) and not just a single figure?
Are the assumptions disclosed (standard, volume, contingency)?
Is a contingency planned and traceable?
A round number without explanation, by contrast, is a warning sign.
11. How do you protect yourself as a building owner?
You have several levers at your disposal. Explicitly agree a tolerance limit in the contract, demand a structured estimate with a stated degree of accuracy and plan in a deliberate contingency. If you need a high degree of certainty, you can agree a clearly defined cost ceiling instead of a mere tolerance.
It is also important to manage changes during construction in a disciplined manner, because every change shifts the basis for the project. The article "What does it cost to build a house?" explains how to create a complete budget.
12. Three practical examples
Practical example 1: Overrun due to calculation error
In the case of a detached house, the final costs were around twenty percent above the detailed cost estimate because necessary work had been forgotten in the calculation. Since the cause lay with the architect and no exception applied, he could not invoke the tolerance.
Practical example 2: Boulders in the subsoil
During excavation, unexpectedly large boulders came to light. The additional costs were considerable but not foreseeable. This unforeseen event was not reflected in the cost estimate and was not covered by the architect's liability.
Practical example 3: Additional costs due to special requests
During the planning phase, a family repeatedly changed the floor plan and the fit-out standard. In the end the original estimate no longer held, not because of a planning error, but because the family had changed the basis themselves. They bore these additional costs themselves.
13. Frequently Asked Questions about Cost Estimation
Is the architect's cost estimate binding?
It is binding within its degree of accuracy, but not an absolute guarantee. If the costs remain within the tolerance and the project stays unchanged, the architect has met his estimate.
What is the tolerance level for a cost estimate?
It depends on the phase. A cost estimate from the preliminary project is around plus/minus twenty percent, a detailed cost estimate from the construction project around plus/minus ten percent. For conversions and renovations, the tolerance is greater.
Is the architect liable for cost overruns?
If the detailed cost estimate is exceeded by more than the agreed tolerance, a breach of contract is presumed. The architect is liable where there is fault, unless the additional costs are due to special requests from the client or unforeseen circumstances.
Can I really hold the architect liable in the event of a cost overrun?
In theory yes, in practice it is difficult. The client must prove fault, an actual loss (less the added value) and that they would otherwise have decided differently. Prevention through clear agreements is more effective.
What is a cost ceiling?
A contractual agreement specifying a maximum amount that the costs must not exceed. Only then is a genuine cost guarantee created, and the architect plans specifically towards this goal.
14. When an independent review is worthwhile
Placing a cost figure correctly and clarifying responsibility in the event of a dispute is difficult for laypersons. An independent review shows which level a figure belongs to, whether the accuracy is right and whether contractual protection makes sense.
raumwert plus reviews cost figures and contracts as a neutral second opinion in the interest of the client. This article explains the principles and does not replace legal advice in individual cases.
How reliable is your cost estimate?
A brief overview shows what you can rely on and where there is leeway.
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