You buy the flat next door to make one large home. Or you own a large old flat and want to turn it into two smaller ones — to let, or for the children. Either way the change is both a building job and a legal one: the wall is one thing, the paperwork another. Do only the first and you will have a home that looks one way on site and another in the cadastre and the title deed.
Merging: one wall, many questions
What kind of wall is it? That is the first question, and the answer is not yours but a structural engineer's. In panel-built and EPK blocks the walls between flats are very often load-bearing. An opening in a load-bearing wall or a floor slab (for a two-storey maisonette) affects the structure — that is no longer routine repair and needs a design and a building permit.
Routine repair without a permit covers only internal alterations that do not affect the structure and do not change the use (§ 5(43) of the Supplementary Provisions of the Spatial Planning Act). More on where that line lies in our guide to renovating a flat in a block.
Whose consent do you need? Load-bearing walls and slabs are common parts of the building (Art. 38 of the Property Act). So for alterations that affect them:
- you need a decision of the general meeting of the condominium and the express written consent of the immediate neighbours (Art. 185(2) of the Spatial Planning Act);
- if the merger takes over common parts — say, a corridor or part of the landing in front of the two flats — you need the consent of all owners with notarised signatures.
How a general meeting is called and what majorities apply is covered in our guide to the general assembly of an apartment block.
Dividing: both new homes have to be real homes
Dividing is harder than merging, because one unit has to become two self-contained units, each of which:
- has its own entrance from the common parts or the street;
- meets the statutory requirements for a dwelling — rooms, a bathroom, natural light, ceiling height;
- has its own installations and meters — electricity, water, heating. That means separate contracts with the power, water and possibly district-heating utilities.
Dividing almost always needs a design and a building permit, and often the Art. 185 consents as well — especially when a new entrance is opened or the risers are touched. If one of the new parts is to become an office or practice, the rules on change of use also apply.
The cadastre: new identifiers
Every self-contained unit in a building has a unique identifier in the cadastre. Two old flats with two identifiers do not automatically become one when you knock down the wall between them.
- Once the works are lawfully completed, the owner commissions a licensed professional to prepare a draft amendment to the cadastral map — the law expressly provides for projects to merge and divide properties (Art. 16(3) of the Cadastre and Property Register Act).
- The Geodesy, Cartography and Cadastre Office records the change; the new unit receives a new identifier and the old ones are closed.
- The new unit plan becomes the basis for future sales, mortgages and taxes.
If the cadastre is not updated while the flat has been merged on site, you have a mismatch — exactly the kind we describe in our guide to errors in the cadastral map.
Don't forget:
- The municipality — file a declaration of the change so the building tax and waste fee are recalculated.
- The condominium — tell the building manager so the condominium register and cost allocation are updated.
- The bank — if there is a mortgage, the change to the unit must be agreed with it.
The effect on price
From our practice:
- Large merged flats have fewer potential buyers. The price per square metre is often lower than for smaller homes in the same building — especially if the merger is not legalised and a buyer cannot get a mortgage.
- Smaller divided flats usually sell and let more easily, and the price per square metre can be higher. But the costs of the design, permit, new installations and cadastre are real, and the result depends on whether both homes are genuinely practical.
- An unlawful alteration almost always reduces the price: the buyer is paying for risk, not for square metres.
If you are buying a merged or divided flat
- Ask for the design and the building permit, and the document confirming the works were completed.
- Compare the cadastral unit plan with the actual layout. Is there one identifier or two?
- Check the title deed — does it describe the unit as it actually is?
- Ask for the general meeting minutes and the neighbours' consents if common parts were affected.
- For a divided flat — are the electricity and water meters and utility accounts separate?
In short
- An opening in a load-bearing wall or slab needs a design and a building permit.
- Where common parts are affected — a general meeting decision and the immediate neighbours' written consent; where common parts are taken over — notarised consent of all owners.
- Merging and dividing are complete only with an amended cadastre and a new identifier.
- A legalised change can add value; an unlawful one almost always takes it away.
If you are thinking of merging or dividing a flat, or are buying one that has already been altered, call us or come into the office. We will review the documents and tell you what is still missing before you spend any money.
This is general information, not legal or technical advice for a specific case. Verified as of October 2026.
