Owning your own home

If I got a tenant in my current place and moved in to a bigger place how do they work out what you can borrow? With only one place, it's just an earnings thing right and LTV but what happens when you already have a mortgage? How will that change the loan likelihood and stuff?

Technically you should inform your lender that you are renting the property out. They may want you to change to a buy to let (BTL) mortgage, usually at a higher rate.

You may also want to remortgage for another reason, that being to free up equity as deposit on your new purchase. I can't remember the numbers off hand but there is usually a cap on borrowing based upon likely rental income.

Your rented property then becomes a business proposition and thus outside of any calculations for your new purchase. Indeed, if you can show a positive income from it, it could be used to enhance your income for borrowing purposes.