Buy to Let
The property must be a residential property and usually on a single tenancy (AST) and have no commercial element to it.
These take up a small percentage of the market and tend to be mostly provided by smaller building societies, the small amount of numbers is reflected with rates being higher than standard buy to lets.
House of Multiple Occupation (HMO)
As a rule of thumb the amount you can borrow is simplified to 4.5 x your total income but this will then be subject to other variables which will be considered such as current debts and outgoings.
If you are thinking of consolidating existing borrowing, you should be aware that if you are extending the term of the debt, you may be increasing the total amount you need to repay.