From the Budget Summary
In order to qualify, applicants must take out a mortgage of at least 80% of the purchase price, or in the case of a self-build, 80% of the valuation approved by the mortgage provider.
Example 4
Deirdre and Evan are planning to self-build their own home in 2017. They estimate that the value
of the home will be €375,000 when completed. For this, provided they have paid sufficient income tax over the previous four years and they are taking out a minimum 80% mortgage, they will qualify for a rebate under the Help to Buy scheme of €18,750.