Offer your thoughts or opinion on the input financial query or topic using your financial background.

Should I buy a house or am I making silly assumptions that I can afford it?
The (interest bearing) mortgage of £300,000 would be SIX times your salary. That's a ratio that was found in Japan, and (I believe) was a main reason for their depressed economy of the past two decades. Even with an interest free loan of nearly £150,000, it would be a huge gamble for someone of your income. Essentially, you are gambling that 1) your income will "grow" into your mortgage, (and that's counting income from renting part of the property) or 2) the house will rise in value, thereby bailing you out. That was a gamble that many Americans took, and lost, in the past ten years. If you do this, you may be one of the "lucky" ones, you may not, but you are really taking your future in your hands.  The American rule of thumb is that your mortgage should be no more than 2.5-3 times income, that is maybe up to £150,000. Perhaps £200,000 if £50,000 or so of that is interest free. But not to the numbers you're talking about.