The Help to Buy: ISA is a savings account for individuals saving towards then buying their first home. You can save up to £1,200 in the first month and then £200 per month thereafter. When your ready to buy your first home the government will add a 25% bonus to whatever amount you’ve saved in between a minimum closing savings balance of £1,600 (for a £400 bonus) and a maximum closing savings balance of £12,000 (for a £3,000 bonus).
The bonus is claimed via your conveyancer (i.e. solicitor) when your ready to buy your first home and is paid towards the cost of completing the purchase rather than towards the deposit.