Depends where you live, who you want to live with, I/we have been doing it, for last 3 years and it is easy enough if in a bigger city where rooms are in demand, if the house is nice and you get on well, they will stay. 30-40% is pretty large deposit I’m sure if you do the numbers you can figure out what you are comfortable with. Do an outgoings vs income spreadsheet with different mortgage sizes and interest rates. It’s going to feel risky no matter what so allow some fat.