For a handyman or attentive homeowner, a construction project represents a physical increase in value. For tax purposes, it's an accounting increase—but only if strict rules are followed. Many homeowners "offer" thousands of euros to the State because they don't know how to distinguish between maintenance and improvements, or because they didn't request the correct invoice.

The 12-Year Rule and Article 51 of the CIRS (Portuguese Income Tax Code): The IRS Code allows deducting expenses related to "property improvement" incurred in the last 12 years from the capital gains tax.

What counts (Technically): Installation of central heating (increases value), replacement of windows with thermal break (increases value), solar panels, construction of a swimming pool or registered garage.

What does NOT count: Repairing a dripping tap (maintenance), garden cleaning, or kitchen furniture (if not built-in/fixed).

The Critical Error with the Taxpayer Identification Number (NIF): A €10,000 invoice for new windows is worth zero for income tax purposes if it only lists "Final Consumer" or if the NIF does not belong to the property owner. Furthermore, the "Improvement" (post-construction) Energy Performance Certificate is the technical proof that completes the deduction process.