Using a LISA and HTB ISA for a new build

ndrew03

Hi everyone, great to be part of the group. My partner and are first time buyers and have our eyes on a new development which is due to be released in August, with completion currently aimed for Spring 2023. have a Lifetime ISA with £15k in it (12k+ 3k gov bonus) and my partner has a Help to Buy ISA with £12k (+ £3k gov bonus), so totalling £30k between us including the gov bonuses. The house that we want is £220k and we will likely try to save another £3k before then to get 85% LTV.

The builders allow an 8week reservation period to allow time for us to sign and return the Building Agreements/Contract, which is when we are expected to pay a 5% deposit via a solicitor, with balance monies due on completion, I have two questions:

1) Can both mine and my partner's government bonuses be used for the deposit, i.e. can we say upfront that we have a £33k deposit even though £6k of this will be gov bonuses? I understand that there is a limitation with the HTB ISA where the bonus money can only be used at completion (the mortgage deposit) rather than the exchange deposit. I was wondering if it would be possible for our solicitor to take 5% of our ISA money for the exchange deposit without affecting the gov bonus at the end.

2) I understand that there is another limitation with the LISA, in that funds must be used within 90 days of the solicitor receiving them (with potential to extend this by a further 60 then 30 days). also understand that new builds are notorious for delays. How can I ensure that my bonus money can still be used towards my deposit if it's likely that completion will be more than 180 days after signing contracts? Given that I could likely sign a contract in August, there is still at least 7 months to March (beginning of Spring, hypothetical date). Could my solicitor request this money at a later stage than the signing of contracts and use the 5% exchange deposit from my partner's HTB ISA instead, and then get the other 10% from both our ISAs closer to the completion date, for example?

3) Lastly, is it a stupid idea altogether to look for a mortgage if offers expire after 6 months? I’m just trying to figure out if this timeline even makes sense in the first place. 

Thank you!