1. Why reconstruction has its own requirement
Three reasons explain why this is a compromise rather than leniency. The existing building limits the solutions.
An existing building already exists and its geometry cannot be redone, so demanding the level of a new building would make many reconstructions impossible. At the same time, reconstruction is the only moment at which improvements can be made cost-effectively, because the scaffolding is already up.
The requirement therefore uses the moment available without demanding the impossible. According to professional sources, substantially reconstructed buildings must meet at least class C energy certificate requirements, and the specific class and its limit value must be checked against the regulation in force, since the scale and the values changed on 1 June 2025.
2. What defines substantial reconstruction
This is the most important question for the project, and no summary answers it. It concerns the scope of the reconstruction.
The requirement applies to a substantially reconstructed building, not to every repair, and the definition comes from the regulation in force. A project near the boundary must be clarified with the local authority before the solution is chosen.
The reason is financial: crossing the threshold of substantial reconstruction brings the energy performance requirement with it, which changes the scope and the budget of the project. This is the same pattern as in the building permit procedure, where the extension percentage determines the type of procedure.
3. What it means procedurally
Two consequences, the second of which binds the construction schedule. They concern the procedure and the certificate.
A calculated energy certificate is prepared for a substantially reconstructed building, as for a new one. According to professional sources, without an energy performance value meeting the requirements a renovated building cannot successfully apply for an occupancy permit.
Energy performance is therefore not advisory in reconstruction but a condition of the procedure, so the solution must be decided at the start of design rather than during the works. Without it the procedure does not move.
4. Why the boundary must be clarified early
Three situations in which late clarity is expensive, and the third is the most common. They concern a change in scope.
| Situation | Consequence |
|---|---|
| The project turns out to be substantial reconstruction only during the procedure | the solution has to change |
| The energy requirement is added later | some works are already done or ordered |
| The client's budget was drawn up without the requirement | a funding shortfall at the worst moment |
The third is the most common because the client does not know to ask. The professional recommendation is to settle this question before signing the design contract.
5. What it means when planning
Three consequences make the scope of a reconstruction a strategic decision rather than merely a technical one. The scope is settled before design.
Comparing two scenarios can be useful, one on each side of the substantial reconstruction threshold, because the threshold changes the requirement, the scope and the budget at once. Where the threshold is crossed, energy performance must be treated from the start of the project, because it affects the solution as a whole.
Summary and four practical rules
Reconstruction carries a separate and lower requirement, because an existing building's geometry cannot be redone, while reconstruction is at the same time the only cost-effective moment for improvements. According to professional sources, substantially reconstructed buildings must meet at least class C, and without an energy performance value meeting the requirements an occupancy permit application does not succeed. The threshold is decisive: crossing it brings a requirement that changes the scope and the budget of the project.
Four rules: clarify the boundary of substantial reconstruction before choosing the solution. Ask the local authority where the project is near the threshold. Budget the energy requirement from the start where the threshold is crossed. Treat the requirement as a condition of the occupancy permit, not as a recommendation.
This article offers professional orientation as at the date of verification. It replaces neither the regulations in force nor the assessment of a competent specialist.