Joining two apartments can create much-needed usable space, but the connection between them must be approached as a structural, condominium and documentation matter—not simply as internal demolition. The first decision is whether you want two legally independent units linked by a doorway, or a permanent merger requiring changes to the horizontal-property title. That choice affects the design work, the consents to obtain and the records to update after construction.
Contents
- Start by separating three decisions that often get mixed together
- Could the wall between the units be structural? Never assume it is not
- What approval should you obtain from the condominium?
- When may internal work be exempt—and when is it not?
- Do not overlook services, meters and the option to reverse the change
- Build the final file before you call the work complete
Start by separating three decisions that often get mixed together
There are three distinct layers to manage. The first is physical: where the connection will sit, what wall will be crossed and whether it contains structure, services or ducts. The second is planning-related: establishing whether the work is an internal alteration exempt from prior municipal control or whether its characteristics require a municipal procedure and design documentation. The third is legal and registry-related: deciding whether each apartment remains an autonomous unit or whether the intention is to alter the horizontal-property arrangement.
An internal door can connect two units owned by the same person without automatically removing their documentary independence. Yet if the aim is to sell, mortgage or let the space later as one home, it is unwise to leave the built reality, title and registers telling different stories. Lisbon City Council sets out the framework for creating and altering horizontal property; use it to prepare the right questions for your architect, notary and condominium manager.
- Obtain the land-registry record and tax record for both units.
- Read the constitutive title and condominium rules.
- Check the use authorisation and available drawings.
- Mark risers, panels, meters and thick walls on the survey.
Could the wall between the units be structural? Never assume it is not
In Lisbon buildings, a thicker wall aligned with walls on floors above or below, or located near beams, columns or stair cores, deserves a technical check before any chasing or demolition. The same applies to apparently lightweight partitions concealing water, drainage, gas, ventilation or shared electrical services. Opening a void without a diagnosis can sever an essential service, weaken a local area or produce a repair that is difficult to carry out properly.
The useful starting point is a measured survey, controlled exploratory openings and review of existing documentation. Where load-bearing elements are involved, the structural design and construction sequence are not minor contracting details: they determine propping, opening dimensions, reinforcement and the safe order of work. The municipal guidance on building works subject to licensing helps frame alterations that should not be treated as ordinary finishing work.
Do not choose the opening location solely from a furniture plan. Compare at least two options: one that shortens daily routes and one that avoids moving services or structure. Shifting a doorway by one metre can sometimes preserve a riser, reduce reinstatement work and leave each flat with a clearer layout should they later be used separately.
What approval should you obtain from the condominium?
Owning both units does not remove the limits created by horizontal property. General water, electricity, heating, air-conditioning, gas and communications installations are commonly shared parts; the title and condominium rules may also impose specific conditions. The Civil Code provides that changing the constitutive title requires agreement from all owners, and that works changing the building’s architectural line or appearance require prior assembly approval under the terms set out there. Read the relevant horizontal-property provisions in the Civil Code published in the Diário da República.
For an internal connection, seek a formal position before appointing the contractor. The notice should explain the scope, show before-and-after plans, state whether structure or shared services are involved, identify the responsible professional and set out dates, hours, lift protection and waste removal. If the merger is permanent and requires an amendment to the title, the minutes should record the necessary agreement precisely; a vague approval for “works in the apartment” does not replace that step.
Do not ask neighbours to sign without documentation. A clear plan and a short note on likely impacts make the decision quicker and reduce reasonable concerns about noise, safety and future responsibilities.
When may internal work be exempt—and when is it not?
In Lisbon, some alterations inside buildings or individual units may be exempt from licensing or prior notice where they do not affect structural stability, façades, building height, roof form or roof covering, and do not involve removing façade tiles. Exemption is not permission to proceed without design work or without checking all other applicable rules. The City Council itself describes these limits in its guidance on public-space occupation for works exempt from prior control.
An opening through a structural wall, a change of use, work to a listed building or work within a specially protected area may take the project out of the straightforward route. It is also important to separate work inside the home from logistics outside it: scaffolding, a skip, material deliveries or reserving street space may require their own permissions even where the internal alteration is exempt.
- Ask the designer for an initial note on the applicable planning route.
- Check whether the building has heritage protection or known constraints.
- Establish whether public-space occupation will be needed.
- Only then set the construction start date.
This order avoids booking labour and materials before documentation and site logistics are resolved.
Do not overlook services, meters and the option to reverse the change
Two joined apartments may still have two electrical panels, two water contracts, two kitchens or two bathrooms. There is no automatic requirement to remove everything, but it is worth deciding deliberately what stays live, what is disconnected and what remains accessible. Closing a meter inside a cupboard without an access panel, concealing valves or turning an electrical panel into storage causes maintenance and safety problems.
Prepare a services drawing showing what will remain. If a kitchen is no longer used, confirm the right way to terminate water, drainage, gas and extraction; an abandoned pipe with a dry trap can create odours. Keep risers and valves accessible where they may serve other floors. If suspended ceilings are used to coordinate new services, plan inspection hatches and ceiling heights from the outset, as covered in our guide to suspended ceilings in renovation.
Also consider future reversal. A connection placed so that it can be closed without rebuilding the whole home, two workable entrance zones and clearly identified services can preserve options without forcing unnecessary duplicate finishes today.
Build the final file before you call the work complete
Construction finishing is not the end of the process. Keep final drawings, the written scope, photographs taken before walls were closed, applicable declarations, invoices, equipment information and condominium resolutions. If horizontal property has been changed, complete the deed and registry updates that match the approved arrangement. If the two units remain separate, the file should clearly state what was built and which services remain independent.
Before handover, walk through a short checklist: does the connection work without clashing with furniture? Are panels and valves accessible? Have shared areas been repaired? Do the final drawings reflect the built work? A decision log from day one makes this stage easier; see our guidance on the renovation kick-off meeting and a three-week plan for a Lisbon renovation.
Bring in a qualified professional before opening the wall where there are signs of structure, shared services, gas, significant cracking or uncertainty about the municipal process. Those issues should never be settled through trial and error on site.
Key takeaways
- Decide first whether you want a doorway between two units or a formal change to horizontal property.
- Do not open a wall between apartments without a technical survey of structure and services.
- Obtain a formal condominium position and keep plans, minutes and final documentation.
- Plan access to meters, valves and risers before closing finishes.
If you are considering joining two units in Lisbon, ask Obra Lisboa for help organising the survey, design work and construction sequence before appointing demolition work.
Sources and further reading
- creating and altering horizontal property
- building works subject to licensing
- public-space occupation for works exempt from prior control
- Civil Code published in the Diário da República
Editorial image; it does not depict an Obra Lisboa project.
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