Help To Buy ISA & Building a home

If the land already has a house on it which you're going to improve to make a better one, you're OK, but not if it's just a field, afaik.

Per the regulations you can get the bonus if you are a 'first time buyer' buying an 'eligible interest in land' which is secured by a legal mortgage.

From definition of 'first time buyer' in the regulations, following the acquisition of interest in the land you have to occupy the land as your only or main residence.

From definition of 'eligible interest in land', it is a legal interest in land situated in UK, solely or jointly owned, described in paras (a) and (b) of the definition of 'residential property owner'.

Paras (a) and (b) of 'residential property owner' say about owning an interest in land:

(a)
is:
(i) in England or Wales:
(a) freehold;
(b) leasehold, where the lease was originally granted for a term certain exceeding 21 years; or
(c) commonhold; or
(ii) in Scotland:
registered or recorded, or would be capable of being registered or recordered, as a right of absolute ownership in the Land Register of Scotland or General Register of Sasines in Scotland, as applicable; or
(iii) in Northern Ireland:
(a) freehold; or
(b) leasehold, where the lease was originally granted for a term certain exceeding 21 years;

and

(b)
comprises a building that is used or suitable for use as a dwelling, or is in the process of being constructed or adapted for such use

So I would assume that if you are buying a 'blank canvas' piece of land that doesn't contain any building at all, let alone one that is used as a dwelling or being constructed/adapted for such use, and you can't make it your main residence, it is a non-starter.

Whereas if you are buying an in-progress barn conversion, or a new-build house that someone else is constructing for you, you're OK.

Presumably the government's thinking is that if you already have enough money to build a house on the land you're acquiring, and you already have somewhere to live while you're building the house, you're less in need of a £3k handout.