Splitting or merging units: what it changes for the whole building

A large flat turned into three studios means three times the traffic, waste and wear — for the same share allocation.

Splitting a unit has become common: a large flat becomes two or three smaller ones, easier to let. It looks strictly private — the owner is remodelling their own home. In practice it affects the whole building, and it is one of the subjects a board too often discovers after the fact.

The principle is simple: what happens inside the unit is the owner’s business, but anything touching the common areas, the share allocation or the building’s legal documents belongs to the meeting.

What requires approval

• Creating a new front door or altering an access from the common areas.
• Cutting through a load-bearing wall, which affects the structure.
• New drainage, ventilation or connections onto the common risers.
• Anything visible on the façade: a new window, grille or vent.
• Amending the schedule of division and the allocation of shares.

These are exactly the points detailed in private works and common areas: a split concentrates almost every case at once.

The share allocation question

This is the most misunderstood point. Splitting a unit does not create extra shares: the original unit’s shares are divided between the new ones. The total stays the same, and the association does not collect more in general charges because one flat became three.

Actual costs, however, rise: three households wear the lift, the common areas, the water supply and the bins more than one. The mismatch is structural, and it is why these operations deserve the board’s attention even when perfectly regular.

What the board can do

• Ask for the item to go on the agenda as soon as a split is contemplated, not after the works.
• Check the amended schedule of division was drawn up by a professional.
• Ensure special charges (lift, heating) are coherently allocated between the new units.
• Check that connections onto common risers were technically approved.
• Update the list of owners and occupants once the operation is complete.

That last point is trivial but decisive: without it, notices go to the wrong address and decisions become challengeable. See the notification rules.

The reverse: merging two units

Merging two adjoining units raises the same questions in reverse: removing a front door, opening between the two, sealing a connection. Shares are added together, and the schedule of division must also be amended.

One point is often forgotten: removing a front door changes the common areas and can affect fire compartmentation. Treat it with fire safety in mind, not only aesthetics.

When the split has already happened

The most frequent case: the board discovers it when new letterboxes appear. It has no sanctioning power, but it can observe, document and alert the manager, the only party able to act for the association.

Regularisation remains possible and common, but never automatic — and changes affecting common areas may have to be undone. Hence the value of an early, dated, photographed report: see the incident log.

Splitting a unit creates no additional shares, but it does create additional occupants: the whole mismatch sits there.

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The neighbouring case, just as frequent: converting a unit into housing.