Why the boundary matters so much
Most technical requirements were designed for new build, where everything is decided before being executed. Their transposition to existing buildings requires its own rules.
Applying them to existing buildings would often be impossible or disproportionate. Raising a ceiling, widening a stair enclosure or reworking a structure to meet a requirement designed for new build can cost more than demolition.
The legislator therefore provided distinct regimes, and that distinction explains most of the exclusions found in technical texts.
Each field draws the boundary its own way
This is what makes the subject difficult, and it is rarely explained. The notion of existing is not defined identically everywhere.
| Field | Criterion used |
|---|---|
| Fire safety | date of the building application, by building type |
| Energy performance | extent of the intervention relative to the envelope |
| Acoustics | date of the permit application |
| Accessibility | nature of the intervention and thresholds |
The same project may therefore be "new" for one field and "existing" for another. This is frequent in major refurbishment, and it is the commonest source of error.
The practical rule follows: the question is asked field by field, never once and for all.
The major renovation threshold
A mechanism present in several fields, with modalities specific to each. It concerns the major renovation threshold.
Where an intervention exceeds a certain threshold, measured as the case may be in percentage of the envelope, in area or by nature of works, it moves into a regime of enhanced requirements, close to that of new build. Crossing the threshold changes the applicable regime.
It is a budget cliff. A project slightly below and one slightly above do not cost the same, although their briefs are nearly identical.
Checking which side of the threshold the project falls on is therefore an estimating step in its own right, and sometimes a decision lever, as the guide on construction costs per m² shows.
What still applies to existing buildings
One misconception deserves correcting: existing buildings are not outside the law. Several bodies of rules continue to apply there.
Three families of rule continue to apply. They concern safety, health and heritage.
Obligations linked to operation. A building receiving the public or workers remains subject to the corresponding requirements, whatever its date of construction.
Obligations linked to transactions. Sale and letting trigger documentary obligations, whatever rules applied to the original construction.
New obligations bearing on the stock. The regions are progressively imposing requirements on existing buildings, notably on energy, a subject covered in the guide on construction costs per m².
That last development is the most significant of recent years. It breaks with the principle that a building compliant when built remains so indefinitely.
The costliest error
It takes two symmetrical forms, and both occur. One overstates the obligations, the other ignores them.
Applying requirements that are not required. Treating an existing-building project as new build produces unjustified and sometimes considerable extra cost, and needlessly complex technical solutions.
Omitting requirements that are required. Assuming existing buildings escape everything leads to discovering obligations late, when they cost most.
The second is graver, but the first is more frequent. It stems from misplaced caution, and it is rarely questioned since it never produces an incident.
What this means for a professional
Four rules.
Ask the question field by field, not once for the whole project.
Identify the applicable reference dates, which differ by field and building type.
Check the position relative to major renovation thresholds, before pricing.
Document the reasoning followed, so as to be able to justify why a requirement was applied or set aside.
This article reflects the position of the rules at the date of checking and serves professional orientation. It does not constitute legal advice and does not replace consulting the applicable texts.