Additional borrowing on mortgage - Timing and Documents

[xyz123]

Hi, we have about 90k mortgage left on a property worth around £450k in London. fixed deal is coming to an end in November.

We are looking to get loft conversion done in March/April next year and will need to borrow additional £35k. Thankfully We both have full time permanent jobs so affordability shouldn't be a problem.

Question is on timing, can i borrow additional money when I remortgage in November and then use it for the loft conversion in March when I need it or do I have to remortgage and then borrow more money in March. I know we will be paying interest for 3/4 months when we don't need money but it will Make it easier to get it all done in one go and also not be possibly on two different rates.

Secondly for additional borrowing, what documents do I need to show? I don't mean payslips etc but more like any planning permission, architecture drawings etc.

Thanks

[xyz123]

Thank you for your replies.