What counts as a hidden defect
One that could not have been noticed on an ordinary inspection and was not disclosed to you.
Cracked plaster you can see usually is not. Damp that appears only in certain conditions, a structural problem behind cladding, or a charge the seller knew about and did not mention, usually is.
The line is not drawn by feel, but by what was visible and what the seller knew.
What can be claimed
Depending on the case: a reduction in price, the cost of putting it right, or rescission of the sale. If the seller knew and stayed silent, their position is worse.
Rescission is sought where the defect is such that, knowing about it, you would not have bought at all. It is not automatic and is judged by how serious the defect is.
Deadlines, and why waiting hurts
Waiting damages you twice over here.
First, there are time limits for bringing a claim and they are not long. Second, proof depends on the state of the property. If you repair it in the meantime, the evidence disappears along with the problem.
So the order is the opposite of instinct: document first, repair second.
What to do straight away
Photographs and video with dates. If possible, a written invitation to the seller to come and see the condition. Quotes or reports from tradesmen on the cause and the cost. For a structural problem, an opinion from a specialist.
Throw nothing away, and do not sign any settlement before checking what you are giving up.
If you bought from a developer
The regime here differs from buying from a private individual, and guarantee periods depend on the type of work. If the problem is in a new building, say so from the start, because the route is different.
If you are abroad
Inspection and documentation can be arranged on site through a representative. Do not leave the property unchecked over the winter if you suspect damp.
What I will need
The notarial deed, the preliminary contract, the listing, correspondence with the seller or agent, the photographs, and anything from tradesmen or specialists. If you have paid for repairs, the invoices too.
This page provides general information and does not constitute legal advice on any specific case. Rights and deadlines depend on the facts and the applicable law.