In short: two separate records and who files
After extending a house, adding a storey, building on the plot or a major rebuild, the property changes in two places. The Catastro, the state cadastre, describes the property for tax purposes: it drives the cadastral value and the IBI property tax. The Land Registry (Registro de la Propiedad) protects rights: exactly what you own and what charges it carries.
The Catastro must be told about the change within two months of completion. You can do it yourself: online at the Sede Electrónica del Catastro with a digital certificate or Cl@ve, at the regional Gerencia, at Puntos de Información Catastral or at the town hall. You are exempt if the notary or land registrar has already passed on the data, or if the town hall has adopted the communication procedure and sends the licence details to the Catastro itself.
At the Land Registry, the extension is recorded with a deed declaring new construction or extension (escritura de declaración de obra nueva o ampliación). A notary drafts the deed, but you can gather the papers, choose the notary and lodge it at the Registry without a middleman.
When it is needed and when not
It is needed when what is recorded about the building changes: floor area, number of storeys, a new structure on the plot or a change of use. Examples: you added a room and enclosed a porch at your house in Castelldefels; you built a loft storey in Sant Just; you merged two Eixample flats into one. In all these cases, without an update the Registry, the Catastro and reality diverge, and it will surface on sale or mortgage: the bank and the notary compare descriptions.
The cadastral declaration, per the Catastro itself, covers new construction, extension, refurbishment or rehabilitation. If you replaced the kitchen and repainted, neither area nor features change and there is nothing to declare. If you are unsure whether your works count, ask at a Punto de Información Catastral before the two months are up.
An old extension built without a licence. If the period in which the council could order demolition has expired, article 28.4 of the Land Act (Royal Legislative Decree 7/2015) allows it to be recorded at the Registry with a certificate of age. That is a separate route and does not amount to legalising it.
Documents and who produces them
For the Catastro, per its portal: a final works certificate endorsed by the professional body, or a deed of completion, or any document proving the completion date; colour photos of each façade; a site plan of the building on the cadastral map; floor plans of each storey with a different layout, as built; a summary of materials and finishes; and cost documents. The certificate and plans come from the technician who supervised the works; you can take the photos and fill in the form yourself.
For the Land Registry, under article 28 of Royal Legislative Decree 7/2015: the council permit or, if the works went ahead on a notice, proof that it was filed and the waiting period passed with no objection; the technician's certificate that the works were finished according to the project; and documents proving compliance with building regulations for handover to users and the authorisations confirming fitness for the intended use and energy efficiency. The notary will give you the exact list for your case.
For an old extension under article 28.4: a certificate from the council or a technician, a descriptive notarial record, or a descriptive and graphic cadastral certificate showing completion on a given date and a description matching the deed.
Step by step
1. Open the licence decision. A town hall that uses the communication procedure with the Catastro must warn you in writing, when granting the licence, that you are exempt from declaring. If that wording is there, you file nothing with the Catastro; if not, assume you must.
2. Get the final works certificate and updated drawings from the technician. 3. Arrange the deed declaring the extension with a notary. Ask for the cadastral reference and graphic description to be included: then the notary and the registrar pass the data to the Catastro and you do not need to declare. 4. Sign and lodge the deed at the Land Registry; the notary usually sends it electronically.
5. If nobody has passed the data on, file the declaration yourself. On the Sede Electrónica del Catastro, log in with a certificate or Cl@ve, choose the declaration of change for new construction, extension or refurbishment, attach the documents and keep the receipt. In person, at the Gerencia, a Punto de Información Catastral or the town hall.
6. Check the outcome: request a new cadastral certificate and a nota simple, and make sure area and description match.
After filing
According to the Catastro portal, the authority has six months to decide on the declaration. The new description changes the cadastral value and with it the IBI: you will see it on the next bills from the town hall.
At the Land Registry, the registrar reviews the documents and records the change; if something is missing you get a formal assessment listing the defects, which you can fix and resubmit. For old works under article 28.4, the registrar notifies the town hall, which must add a margin note stating the building's planning status.
Keep the full set: licence, completion certificate, deed and cadastral certificate. That is what a buyer and their bank will ask for when you sell.
Common mistakes
Assuming the licence updates everything. A licence records nothing at the Registry, and it only reaches the Catastro where the town hall uses the communication procedure.
Waiting until you sell. Ten years later the supervising technician is harder to find, and the sale stalls until the notary reconciles the description.
Giving the Catastro design drawings instead of as-built ones. They want plans of what was actually built.
Confusing recording an old extension with legalising it. A record under article 28.4 carries a note on its planning status and does not make it lawful.
Not checking the result. An area error on the cadastral certificate affects your IBI and valuations for years.
Before you file
Forms and procedures change, so check the current procedures section of the Catastro portal and confirm the Registry list with your notary before filing. If you would rather not do it yourself, BCN YARD can prepare the works documentation along with the construction itself.