Help to Buy - self build Q?

I'm trying to understand the timeline for applying for HTB in my own case but I can't seem to locate a specific answer.

I was working abroad so currently only qualify for about half the 30k from PAYE from 2024.

If I wait until January 2026 to apply I'll be eligible for the 30k.

What I'm trying to understand is, what is the cut off point for being eligible? As in, I obviously want to wait to get the whole 30k but I already know the site I'll be buying (local, not on the market etc) and don't want to wait until 2026 to start working.

If I need the mortgage to purchase the site in, say August 2025, does that mean I cannot wait to apply for HTB in January 2026?

Or if I managed to buy the site myself (might be possible later in the year but would be leaving myself with nada 😅), and then paid for plans/site work etc myself, then drawdown the mortgage to start actual building work in January 2026, can I apply in January once I've submit my tax return to include 2025.

Apologies this is probably not every well explained, but I'd rather get the total amount but also don't know if its worth delaying my project by 5/6 months?

Anyone an idea?