Anyone sought permission for home improvements with Help to Buy Charge on property?

[Bananamanfan]

We bought a large 3 bed semi (new build) under Help to Buy 4 years ago. We have DS1 age 20 and DD 7 & DS2 5. DD & DS2 are sharing a room, DD will be 8 soon and wants her own space.
We had quotes for a loft conversion, which we are happy with. We had a plan with our mortgage advisor to remortgage to pay off the H2B and cover the loft conversion. In the few months since that plan, our house value has dropped by 10% and we would have to borrow over 90% of the current house value to be able to do it.
Does anyone have any experience of applying to borrow against the house with a H2B charge in place? (Sought permission from the relevant HA).

Sorry, I did put paragraphs in, but they're not very clear.

If you were to get permission (which I believe is unlikely anyway) wouldn't you end up costing yourselves money as any extra value thanks to that loft would need the % paid over to HTB too? I'd be taking advantage of having to pay back less at this stage and pay the loan off, postponing the conversion if needed. Could you not split any of the rooms with a stud wall as a temporary measure?

Yes all of that is true. We don't have any spare windows, unfortunately. Thanks for your reply.

If you have some amount of cash to spare, could retrofitting a window not be an option if the space is workable otherwise? Do you have any space in the garden where you could put a playhouse/den just to give her somewhere other than the room to hang out?

Hopefully someone with relevant experience will come along, I know the question has been asked on Martin Lewis' site previously so you're not alone even if in a minority!