I’m not sure these properties can be grouped by price and days alone. Which Utrecht neighbourhood boundaries are you using, and are the buildings comparable in condition and permitted use? Mixed-use buyers can also face different financing constraints, while uncertainty over the tax treatment...
What does “Porto” cover in your sample? If some homes are inside the city boundary and others are simply marketed under the wider Porto label, the typical figure could hide very different micro-markets. I’d also split renovated homes from those needing work before drawing a conclusion about vacancy.
What condition is the flat actually in when the work starts? “New-build” could mean completed surfaces or something closer to a fit-out, and that changes the risk considerably. I’d also establish whether plumbing points stay where they are and whether bathroom moisture documentation and service...
A practical order would be: verify the contract and timing, narrow the comparisons to similar townhouses, estimate vacancy and refurbishment exposure, then choose a defensible adjustment. After that, send a calm written explanation and invite the tenant to discuss it.
I agree with not leading...
I’d add a legal caveat to that sequence: confirm first whether an increase is permitted at this point and what form and notice are required under the contract and the rules applying in Portugal. An informal conversation is useful, but it should not accidentally replace any required written step...