Many lenders will approve your mortgage application if you can afford your repayments at the assessment rate and have enough money left over to pay your current debts and living expenses. In other words you have no spare funds left after meeting your commitments.
Other lenders will require you to have spare funds left over after paying your debts. This is known as a buffer.
The lenders include this buffer because if your situation changes you can still afford the debt and any significant interest rate rises.
If your bank uses a buffer then it will greatly reduce how much you can borrow for a house.