Should I get a mortgage with my spare...

[Katja122]

...money? I have been working for a year as a teacher earning £21000. I live with my fiance - he owns the house with only a small mortgage. I recently worked out that I have around £600 per month spare. We don't really need to move - like the house and it's worth a lot more than the mortgage. Should I think about buying a house in my name, either to let or sell (my boyfriend is a joiner and could do any work)? OR should I just be saving the money?