Renting out a room in your own home, whether to supplement your income or to share costs, creates a tax obligation that many people either don't know about or underestimate, especially when the arrangement is an informal one between acquaintances.
This income is also taxed as real estate capital income
Just like renting out an entire home, renting out a room in your own primary residence generates income subject to IRPF as real estate capital income, regardless of whether the arrangement is informal or there's no written rental contract in place. The informality of the agreement does not exempt you from the tax obligation.
How proportional deductible expenses are calculated
Unlike renting out the whole home, when you only rent part of it, deductible expenses (utilities, property tax (IBI), community fees, mortgage interest if applicable) must be prorated according to the proportion of the home actually devoted to the rental, usually based on the square meters of the room relative to the total home, or another reasonable allocation criterion if it better reflects actual use.
The reduction for renting a primary residence, with caveats
The specific reduction for renting out a primary residence (already covered in another article) can also apply to this situation if the rented room constitutes the tenant's primary residence, though it's worth checking the specific requirements currently in force, since the rules for this reduction have been adjusted in recent years and may require specific conditions that aren't always present when renting out a single room.
Why correctly declaring this income matters more than it seems
Although the amount from renting out a room is usually modest compared to renting an entire home, failing to declare it is still a tax offense, with the risk of penalties if the tax authorities detect the undeclared income (for example, through recurring bank transfers from a third party, increasingly easy to detect through automated data cross-checks).
The difference from sharing costs between roommates with no profit motive
It's important to distinguish between an actual rental (with an amount that generates a profit or at least covers the cost with some margin) and a simple proportional split of shared expenses between people living together with no profit motive (such as roommates who split rent and utilities equally with no one profiting from it), a situation that, in principle, does not generate the same kind of income subject to taxation as a landlord.
Keep a clear record of income and expenses
If you rent out a room in your home, it's worth keeping a clear record of the income received and the prorated associated expenses, so you can correctly declare this income on your next tax return.
Compare this option against other alternatives
Our rent vs buy calculator can help you assess, from a broader perspective, the profitability of different strategies related to your home.