Q I'm thinking of renting out my home and buying a second house in the near future. My current house is worth around 105,000, the mortgage is 82,000 and I think I could get around 475 a month in rent for it. I'm looking to buy a second property with my girlfriend for around 150,000. I earn 30,000 a year and she earns 17,000 a year. MW

A Your plan to rent out your house while buying another property with your girlfriend seems to work – but only up to a point. Because you don't need to borrow more than 80% of the value of the property you could convert your current mortgage to a buy-to-let mortgage. If you wanted to borrow more than 80% of the value of your property, buy-to-let would not be an option because 80% is the most a buy-to-let lender will lend.

Assuming you went for an interest-only buy-to-let mortgage at the cheapest rate you could get for borrowing 80% of the property's value, currently 5%, your monthly mortgage payments would be 340. The rent of 475 would cover the mortgage payment by more than the 125% required by most buy-to-let lenders, which makes a buy-to-let mortgage look feasible in your case.

However, a buy-to-let lender won't take your word for it that you will get rent of 475 a month; it will get a professional valuation on the rent your property could achieve. If the professional's figure is less than the 425 needed to cover the mortgage payment by 125%, your application for a buy-to-let mortgage would probably be turned down.

The real problems start with your plan to buy a house for 150,000 with your girlfriend. On paper, if you could borrow a not-unheard-of 3.25 times your joint earnings of 47,000, you could get a mortgage of 152,750. But you are unlikely to be able to do so because no mortgage lender these days is prepared to lend the full amount of the price of a property. So the big flaw in your plan is the lack of a deposit for the jointly owned property. For your plan to work, you need to find at least 10% of the value of the property you want to buy to put down as a cash deposit — although a deposit of 15% would be better because the interest rate you would pay on the mortgage would be lower.

For some people considering letting a current home while buying another one, there is the option of raising cash for a deposit on the second property by increasing the loan on the current home when converting it to a buy-to-let mortgage. However, as already explained, your current mortgage is nearly up to the maximum 80% that buy-to-let lenders will lend. So increasing your current mortgage to raise a deposit is not an option in your case. Instead, you might want to think about letting your girlfriend move in with you in your current home while you save up for a cash deposit of at least 15,000 for a jointly-owned home.

Muddled about mortgages? Concerned about conveyancing? Email your homebuying and borrowing worries to Virginia Wallis at virginia.wallis.freelance@guardian.co.uk