You want to live in a house yourself and rent out a room or another separate part. In the purchase calculation the expected income covers part of the loan burden. Before this amount makes the project appear feasible, your housing needs, the actual possibility of use and the cash flow must fit together.
This article does not explain an allowable scope of letting and does not calculate taxes. Structural, legal and contractual prerequisites must be clarified professionally for the specific property. An area separated on the floor plan alone is not a complete proof for this.
Describe your own living solution honestly first
Specify which area is to be given up and which shared areas or routines would be affected by this. If the room will later be needed for a child, work or care, that possible period belongs in the planning. The same room cannot at the same time be a permanent reserve space for yourself and a secured basis for letting.
Also check which actual additional expenses and tasks would belong to the intended use. The expected agreed rent is not identical to a fully available net surplus without further allocation.
Three hypothetical financing perspectives
Without the additional letting there is a monthly shortfall of 350 euros in the fictional household case. For a possible tenancy agreement 500 euros of income are assumed. Further already outlined disbursements for the letting amount to 100 euros. Under these assumptions alone the calculation would improve by 400 euros and end with a remainder of 50 euros.
In the second variant there are no receipts for three months while the private basic charges remain. For the original shortfall 1.050 euros would then be needed from other means before further letting-related expenses are considered. In the third variant the area is required again for own use later; the ongoing financing plan would then have to work without the additional surplus.
These variants do not make a statement about typical lettability or a permitted notice to quit. They show the dependence on an actual use, which in your own life plan need not remain unchanged.
Ask the bank question and the personal question separately
Ask how the lending institution will take the planned income into account. A positive crediting does not replace your own check whether you actually want to live with another person in the house and can bear the plan if the income is missing.
Note which documents and confirmations are still missing. An optimistic rent estimate does not become a reliable source of funds because the preferred house would otherwise lie outside the bearable framework. The decision is comprehensible if the financing impact of partial letting remains explicitly visible and at the same time a personal residential and contingency plan exists.