What renovation changes
In new build, the building is the outcome of the project. In renovation, it is the input. It exists, it has a history, it has been altered, and part of what it contains is not visible.
Uncertainty therefore bears not only on what will be done, but also on what is already there. That is why new-build methods do not transpose.
Five consequences follow, each the subject of an article. They touch the bill of quantities, the price, the contract and the calendar.
| Area | In new build | In renovation |
|---|---|---|
| Survey | unnecessary | indispensable preliminary |
| Measurement | determined quantities | partly estimated quantities |
| Budget | a firm price is conceivable | a contingency is necessary |
| Contract | defined object | partly unknown object |
| Programme | predictable sequence | probable interruptions |
Structure is the major risk, because it governs everything else, is discovered late and costs disproportionately. A structural discovery stops the project, whereas a services problem can be worked around.
On budget, the difference is one of kind. In new build, a budget is exceeded through choices. In renovation, it is also exceeded through discoveries, independently of any decision. A price without a contingency is not dishonest: it covers a scope not fully known.
These points are developed in the branch on what renovation changes. The differences from new build are set out there.
What to know before starting
A property sold comes with documents. Those documents do not suffice to prepare a project.
Two families of survey coexist. Transaction surveys inform the purchaser and are generally non-destructive. Works surveys prepare the intervention and are often destructive.
The case of asbestos
The use of asbestos was prohibited from 2001, making that year the pivotal date for all obligations.
The regions diverge sharply. One region has required a certificate on transfer of ownership since late 2022; the other two require nothing to date, while considering changes.
The essential distinction lies elsewhere: the sale certificate is non-destructive and does not look inside walls. It therefore does not suffice before works, where a complementary destructive investigation is needed. Firms employing workers face a separate obligation under welfare-at-work rules.
The other surveys
The energy certificate has changed in nature. It used to inform; it now triggers obligations. Its most misleading limit is that it rests partly on default values absent proof, penalising a well-insulated but undocumented property.
Electrical inspection is federal, therefore identical across the three regions, which makes it an exception. Any substantial modification of an installation triggers it, and the outcome may require works beyond the planned scope.
These points are developed in the branch on surveys and preliminaries. The obligations are listed there by region.
The tax regime
The gap between full and reduced rates is one of the heaviest items in a renovation budget. It runs into thousands of euros on an ordinary project.
Four cumulative conditions: a dwelling first occupied at least ten years ago, use as private housing after works, supply to a final consumer, and a detailed statement on the invoice.
Two dates are compared for age: first occupation and the date of the first invoice, not the quotation. On a property near the threshold, deferring the first invoice may be worth several thousand euros.
The 2022 change
The certificate signed by the client has gone, replaced by a detailed statement carried by the contractor on their invoice.
The client therefore supplies nothing further, but remains responsible. If they do not contest in writing within the period allowed while the conditions are not met, they may be liable for the additional tax.
That mechanism is riskier than the old one because it is more discreet. The client signs nothing, the statement sits on a rarely reread financial document, and the period runs while works proceed. Hence the rule: address the question at quotation stage, not at invoicing.
Do not confuse two regimes
Demolition and rebuild falls under distinct social conditions, with a floor area ceiling, a sole-and-own-dwelling requirement and a domicile obligation.
One recent trap deserves attention: the floor area ceiling differs between ordered works and a sale. The same dwelling may therefore qualify in one case and not the other.
These points are developed in the branch on the tax regime. The conditions for the reduced rate are set out there.
What the law requires of existing buildings
A building compliant when constructed remained so indefinitely. That principle no longer holds.
The decisive difference between regions concerns who is covered. In Flanders the obligation arises from a transfer: an owner who neither sells nor buys remains outside the scheme. In Brussels it applies to all owners, on calendar deadlines. Wallonia combines both approaches.
The Flemish scheme is the oldest and best documented. The purchaser of a poorly rated dwelling must reach a minimum level, within a period extended from five to six years in early 2026, on pain of an administrative fine.
The notion of transfer is wide: sale, gift, long lease, building rights, certain forms of contribution. A gratuitous transfer may therefore trigger the obligation.
Practical consequence for a purchaser: the rating becomes a price factor, and the cost of upgrading must be estimated before commitment.
These points are developed in the branch on obligations on existing buildings. The regional pathways are compared there.
Contracting for uncertainty
A renovation contract must say what a construction contract need not. It must organise how discoveries are handled.
Four clauses answer the same question: who bears the gap between what was assumed and what is found.
The handling of discoveries, with a definition resting on prior verifiability, an obligation to stop, a procedure and above all a default decision rule, almost always absent yet alone capable of preventing a project stalling with nobody bearing the cost.
The variation process, whose governing principle covers everything: no execution without written acceptance.
The regime for interruptions, the most frequent cause in renovation being a wait for a decision, and the one contracts address least.
The status of assumptions, which makes their disproof contractual rather than arguable.
These clauses are drafted beforehand, never during. After a discovery, positions are fixed and the client has few alternatives.
These points are developed in the branch on the renovation contract. The decisive clauses are listed there.
A framework in motion
Three developments decisive for anyone starting a project today. They touch the regional obligations and the tax rules.
The Walloon transitional support scheme ends on 30 September 2026, the date for submitting a complete application rather than starting works. The following scheme will rest more on financing.
Taxation ceases to be technology-neutral, with a reduced rate extended to heat pumps and the full rate applied to certain fossil-fuel installations.
All three regions are shifting from grant to loan, which makes financial structuring a skill in its own right.
These points are followed in the article on developments to monitor. The calendars are kept up to date there.
The method, in summary
Six rules, applicable to any renovation in Belgium. They order the project from survey to invoice.
1. Investigate before pricing, and gather what exists before commissioning surveys.
2. Distinguish measured, assumed and conditional quantities, and write down the assumptions.
3. Provide an explicit contingency, calibrated by risk analysis rather than a standard percentage.
4. Check tax eligibility at quotation stage, not at invoicing.
5. Check regional obligations at source, this area having seen several recent revisions.
6. Write the handling of discoveries before a discovery occurs.
The detailed methods appear in the articles on the preliminary file and on checking eligibility for reduced VAT. The documents to gather are listed there.
This guide reflects rules and professional methods at the date of checking and serves as orientation. It does not replace technical, tax or legal advice, or consultation of the competent administrations.