You’ll need at least 20% of the value of your property as security for your existing loan.  To calculate how much usable equity you have in your home, use the following calculation:
Current value of your home x 0.8 = 80% of the value of your home
80% value of your home – your remaining loan debt = the amount of free usable equity you currently have in your home.
This is the amount that a lender may give you as a cash-out option if you refinance your loan and add to your current loan with a top-up.  Use Savvy’s calculator to find out how long it will take you to repay your loan so you can experiment with different repayments until you find a level which fits in with your existing commitments.