You can use your Help to Buy ISA savings to pay for the deposit. However, you cannot use the government cash bonus since you will only receive that when you complete the buying process. Sometimes, the amount of money you have in your Help to Buy ISA account is not enough for the deposit without the bonus. In this case, you can try asking the solicitor if it’s possible to pay a lower deposit at first. You can then pay for the rest of it when you received the bonus and completed the purchase. If the seller does not allow this, you will have to find other sources of funds. Then when you complete your purchase, you can use the cash bonus for repayment of your mortgage. When you complete your purchase, the money goes directly to the mortgage lender through your solicitor.