Anyone building or extensively renovating in Austria often hears from developers and construction companies that the fixed price is final and there will be no surprises. In practice, however, many builders experience supplementary claims that significantly exceed the originally agreed amount. The decisive question then is not whether a fixed price was agreed, but whether the specific additional claim is legally justified.

Before signing a construction contract or responding to a supplementary invoice, it’s worth examining the underlying scope of work, the works contract (Werkvertrag), and the additional work actually carried out. Simon Immobilien helps assess such documents as part of its property advisory services, but this does not replace a legal or judicial review of the individual case — ideally, bring the construction contract, the scope of work, and all previous supplementary letters to a consultation.

Key Points at a Glance

1. A fixed price binds both contracting parties to a fixed total amount for the contractually defined service, not for every conceivable construction task.
2. Extra costs are permissible when services fall outside the original scope of work, the client requests changes, or unforeseeable circumstances undermine the basis of calculation.
3. The Austrian General Civil Code (ABGB) governs the principles of fixed-price agreements in works contracts under Section 1170a, supplemented by Supreme Court (OGH) case law.
4. ÖNORM B 2110, a widely used Austrian construction contract standard, is agreed as the contractual basis in many construction contracts and contains its own rules on additional and reduced work.
5. Documenting supplementary claims in writing early and clarifying changes in scope before execution avoids most disputes.
6. If there is uncertainty about the legitimacy of a supplementary claim, expert legal advice before payment is recommended.

Fixed Price and Supplementary Claims: What This Topic Is About

A fixed price is frequently agreed in Austria for single-family homes, renovations, and turnkey construction projects because it promises budget certainty. Builders deliberately choose this pricing model to shift the risk of rising material costs or unclear quantity estimates onto the contractor. However, this very expectation leads to misunderstandings as soon as additional work arises during construction or the original plan changes.

This topic is relevant for private builders as well as for buyers of terraced houses or semi-detached houses who sign a developer contract under the Austrian Property Developer Contract Act (Bauträgervertragsgesetz), and for owners commissioning a renovation or extension. The legal framework arises primarily from the ABGB as works contract law, from ÖNORM B 2110 where contractually agreed, and — for developer projects — from the Property Developer Contract Act with its rules on installment payments and security. In practice, the exact wording of the scope of work usually determines whether additional work is still covered by the fixed price or must be paid separately.

A typical starting situation looks like this: the client receives an offer with a total amount, signs the contract believing everything is covered, and later discovers that soil surveys, utility connections, or certain fittings are billed separately. Whether this is lawful depends on the original scope of work — not on either party’s wishful thinking.

Basic Concepts: Fixed Price, Unit Price, and Scope of Work

What a Fixed Price Means Legally

A fixed price within the meaning of Austrian works contract law refers to an agreement on a fixed total fee for a service specifically described in the contract. The contractor bears the risk that their calculation works out for exactly this service. If the scope of work remains unchanged, they generally cannot make a supplementary claim, even if actual costs turn out higher than calculated. This applies, for example, to increased material prices, unless an indexation clause was agreed.

The counterpart is the unit-price contract, where individual items are billed according to the quantity actually delivered. Many construction contracts combine both systems: a fixed price for the main portion, and unit prices for certain trades where quantities remain uncertain, such as earthworks or foundations.

The Scope of Work as the Actual Benchmark

The fixed price always relates to a defined service. This definition comes from the scope of work, the construction description, plans, and technical documents that form part of the contract. If a task is not included there, it constitutes additional work that generally must be paid for separately. If it is included but described imprecisely, interpretation follows the rules of the ABGB regarding how a reasonable contracting party could have understood the wording.

A common misconception is equating the fixed price with a kind of comprehensive insurance against any type of extra cost. This is not accurate. The fixed price protects against extra costs arising from the contractor’s area of risk — such as miscalculation, unexpected additional effort for the agreed service, or increased labor costs. It does not protect against extra costs arising from the client’s change requests, regulatory requirements, or unforeseeable ground conditions, provided these were not part of the original scope of work.

ÖNORM B 2110 and Its Significance

Many Austrian construction contracts explicitly reference ÖNORM B 2110 as their contractual basis. This standard contains detailed rules on changes in scope, price adjustments for additional or reduced work, and deadlines for notifying such changes. If ÖNORM is agreed, the contractor generally must notify a change in scope in a timely manner and may not simply present a supplementary invoice at the end of the project. If there is no reference to ÖNORM, the general rules of the ABGB apply, which are less detailed but equally binding.

What Actually Matters When Assessing Extra Costs

Whether a supplementary claim is justified depends on several factors that are examined closely in a dispute. First, it matters whether the additional service was described in the original contract or not. Equally important is who initiated the change — the client through a special request, or the contractor through a necessary technical adjustment. Finally, it matters whether the extra costs are based on a circumstance that was foreseeable for both parties at the time of contract conclusion or only emerged later.

The following situations regularly lead to justified extra costs in practice:

• Change requests from the client after the contract was concluded, such as a larger window area, an additional room, or higher-quality fittings than originally agreed.
• Services that were simply missing from plans or the construction description, such as soil replacement due to insufficient load-bearing capacity, provided a soil survey was not part of the contract.
• Regulatory requirements that only arise during the building permit process, such as additional fire protection measures or changed energy efficiency requirements.
• Unforeseeable ground conditions such as bedrock, groundwater, or contaminated soil that could not have been identified through careful preliminary investigation.
• Delays caused by the client themselves, for example through late decisions on fitting details, provided these result in extra costs for the contractor.

By contrast, claims are not justified when they simply result from the contractor’s overly tight calculation, from increased material prices without a contractual indexation clause, or from services that were clearly part of the original description but were overlooked by the contractor. Anyone unsure whether a particular item is still covered by the original construction description should compare it against the written scope of work and the approved submission plans before paying or objecting.

Process for a Supplementary Claim: Step by Step

1. Review the supplementary letter: Examine the claim together with the original construction contract and scope of work, ideally within a few days of receipt.
2. Compare with plans and construction description: Clarify whether the disputed service was included in the original contract scope — this step takes one to two weeks depending on complexity.
3. Consult with the construction company: Request a written justification for the extra costs and a comprehensible calculation of the additional work.
4. Involve a construction-technical or legal expert: For disputed or larger amounts, an assessment by a master builder, chartered engineer (Ziviltechniker), or a lawyer specializing in construction law is advisable.
5. Written response to the construction company: Document your position in writing, even if you accept parts of the claim and reject others.
6. If the dispute continues: Consider an out-of-court settlement, such as mediation or an arbitration procedure through the Austrian Chamber of Commerce, before considering legal action.
7. Judicial clarification as a last resort: If disagreement over larger sums persists, a lawsuit before the competent district or regional court remains an option, usually accompanied by a construction-technical expert opinion.

These steps do not replace legal advice tailored to individual cases, but they provide guidance on a sensible sequence of action — avoiding both hasty payment and missed deadlines for legitimate claims.

Case Example: Comparing Two Supplementary Claims

To illustrate, here is a calculation example with figures marked as indicative reference values, without claiming to reflect an actual real-world calculation. A client has agreed a fixed price of 380,000 euros with a construction company for a turnkey single-family home — a reference value for a mid-sized project in the area surrounding Lower Austria.

In the first scenario, the construction company issues a supplementary invoice for 12,000 euros after construction begins, because bedrock was encountered during excavation that was not identified in the soil survey report and whose removal required additional machinery. Since this circumstance was not identifiable at the time the contract was concluded and lies outside the original scope of work, the supplementary claim is, in principle, justified in this example, provided the extra costs are documented in a comprehensible manner.

In the second scenario, the same construction company additionally demands 6,500 euros because the windows had to be purchased at a higher price than calculated. These extra costs arise solely from the contractor’s misjudgment of market prices and relate to a service that was clearly part of the original fixed price. In this case, the contractor bears the calculation risk, and the supplementary claim is generally not justified.

The comparison shows: it is not the amount that matters, but the assignment to the original scope of work and the respective area of risk.

Type of Claim	Reason for Extra Costs	Likely Justification
Unforeseen ground conditions	Bedrock, groundwater, not identifiable in soil survey	tends to be justified
Client change request	Larger windows, additional room, different fittings	justified, billed separately
Additional regulatory requirement	Fire protection, energy certificate requirement after submission	tends to be justified
Contractor miscalculation	Underestimated material prices, incorrect quantity calculations	tends not to be justified
Overlooked contractual service	Item was included in construction description but was forgotten	not justified

Common Mistakes and How to Avoid Them

A common mistake is signing a construction contract with a vague scope of work, expecting that everything essential is included anyway. The less precise the construction description, the greater the scope for later supplementary claims. It is equally risky to agree to change requests verbally without recording them in writing along with a price — in a dispute, a verbal agreement is difficult to prove.

Another typical mistake is paying supplementary invoices too hastily in order not to jeopardize construction progress. Anyone who pays without checking legitimacy often loses their negotiating position for later corrections. The opposite is similarly problematic: reflexively rejecting every supplementary claim risks construction delays and conflicts that could be avoided through an objective, documented review.

The importance of the soil survey report is also frequently underestimated. Anyone building without a meaningful survey report bears a higher risk of later facing justified supplementary claims due to unexpected ground conditions. Finally, many contracts lack a clear provision on indexation or fixed-price guarantees for increases in material and labor costs — without such a clause, it often remains unclear who bears the risk of future price increases.

Checklist: Correctly Assessing a Fixed Price in a Construction Contract

• Review the scope of work and construction description completely and in detail before signing the contract.
• Clarify whether ÖNORM B 2110 is agreed as the contractual basis.
• Obtain a soil survey report and make it part of the contract.
• Agree change requests exclusively in writing, including the price.
• Always compare supplementary letters against the original scope of work.
• Tie payments to construction progress and the service actually rendered, not solely to deadlines.
• Seek construction-technical or legal advice early when uncertainties arise, rather than reacting only at the end of construction.
• Specifically review indexation clauses and provisions on price adjustment in the contract.

Frequently Asked Questions

Can a construction company demand extra costs if material prices rise?

Generally not, if a pure fixed price was agreed without an indexation clause — in that case, the contractor bears the price risk. If the contract contains an indexation or price-adjustment clause, an adjustment may be permissible according to the rules set out there. The exact contract wording is always decisive.

What happens if I, as the client, want changes during construction?

Change requests are considered additional work outside the original fixed price and generally must be paid for separately. It is advisable to agree on such requests in writing, including the price, before execution, to avoid later disputes over the amount.

Do I have to pay a supplementary invoice immediately?

No, there is no automatic obligation to pay immediately. It is advisable to first review the claim and, if necessary, object in writing before making a payment — however, contractual payment deadlines should be observed.

What role does ÖNORM B 2110 play in supplementary claims?

If the standard is contractually agreed, it regulates, among other things, how and when changes in scope must be notified and how the price changes as a result. Without an explicit reference in the contract, it does not apply automatically, and the general rules of the ABGB apply instead.

Does the same apply to a developer contract as to a classic construction contract?

For developer projects, the Property Developer Contract Act additionally applies, which specifically regulates installment payments and security. However, the principles regarding fixed price and scope of work are fundamentally similar to those in an individual construction contract.

When should I involve a lawyer?

For larger disputed amounts, repeated supplementary claims, or when an out-of-court resolution with the construction company fails, a legal assessment is advisable before deadlines pass or payments are made.

How Simon Immobilien Can Help

When it comes to assessing construction contracts, scopes of work, and how supplementary costs affect a property’s value, Simon Immobilien brings market and property expertise from numerous construction and renovation projects — detailed legal review, however, remains the domain of lawyers and expert assessors. Anyone considering buying into a developer project or wanting to have an existing property with an ongoing construction contract evaluated can find an overview via Simon Immobilien’s current property listings, complemented by regional location guides and further articles in the property advice section. For a personal consultation, the Simon Immobilien contact page is available.