Gearing
Just because you are not paying off your mortgage debt it does not mean that you cannot exploit the value of your share.

As stated before, you need to (generally) provide at least a 15% deposit for a buy to let property.

Also stated earlier was the fact that if house prices increase then the bit you own is worth more too. Provided you are meeting your mortgage payments, lenders will let you use your portion of the property, or "equity", to borrow more money.

As long as you ensure the rental income sums add up, you can use that newly released money as deposit for another buy to let property.

And if that property also increases in value, you can remortgage and release money again and buy a third property. In this way, in a rising market, it is possible to finance a string of buy to let properties without risking more of your own money than the first deposit leading you down the road of owning multiple properties and then well on your way to becoming a property tycoon!

Gearing (increasing the value of your investment through gearing) allows you to buy much more than you can "afford" in pure cash terms and in that sense it allows you to make a large return on a small stake. However you must be aware that it's not just profits which can be magnified, but losses too!