Short-term rentals, largely to tourists, offer the most lucrative, immediate reward. But recent crackdowns on the practice are making it much harder to earn money without going to the extensive trouble.
Arduous and expensive registration processes, health and safety requirements, and huge bankruptcy-inducing fines for non-compliance mean now is perhaps not the best time to jump on that particular gravy train.
But the long-term rental market shows few signs of slowing down. In major cities and along the coast, rents have shot up by 10 per cent this year, earning thousands of landlords a small fortune.
The key question is, how long will it last? Carme Trilla, president of the Metropolitan Observatory of Housing, has said that rise in rents has "hit a roof". "There will be a bottleneck in price rises because it is impossible to have sufficient demand with the current rents," she said.
She was referring to Barcelona, where the average rent now stands at €845 per month, but it should only be a matter of time before rents across the country begin to stabilise.
That still offers a strong window of opportunity for those looking to capitalise on rent rises while they can.