If you want to sell within the first 5 years, you'll only need to repay exactly what you borrowed. However, selling in the middle of your fixed period would incur early repayment charges on your main mortgage.
If you want to sell after your first 5 years, the amount you'll need to repay will depend on the value of your home.
The table below shows some hypotheticals as guidance. This is assuming constant house price growth of +2% per year, that you don't repay any of the boost and that you buy the home for £200,000.
End of year | Assumed house price growth | Property value | Your equity and mortgage | Boost balance |
Year 1 | +2% | £200,000 | £170,000 | £30,000 |
Year 2 | +2% | £204,000 | £174,000 | £30,000 |
Year 3 | +2% | £208,080 | £178,080 | £30,000 |
Year 4 | +2% | £212,242 | £182,242 | £30,000 |
Year 5 | +2% | £216,486 | £186,486 | £30,000 |
Year 6 | +2% | £220,816 | £187,694 | £33,122 |
Year 7 | +2% | £225,232 | £191,447 | £33,785 |
Year 8 | +2% | £229,737 | £195,276 | £34,461 |