Confused!?! Offset Mortgage, house extension

two8tone

Newbie here, I would consider myself fairly savvy in searching for products but I’m at a loss on the questions below. After visiting high street bank, researching here and forums elsewhere I’ve not tracked down conclusive answers.
Any help greatly appreciated;

1. Lack of information comparing Offset Mortgage
MSE Best Buys comparison tool does not appear to include offset, other comparison sites do not ‘’compare’’ the rates and fees, currently I’ve researched the main banks/building societies still interested in new offset accounts and manually compared the costs myself.

2. Moving to an Offset Mortgage, equity release
Once my current fix product ends and moves to variable rate mortgage, can I simply push my LTV up to 75% from its current 50% for a new re-mortgage application and move the released capital into the linked savings account (in addition to savings currently held elsewhere), to withdraw as and when needed for house extension?
I’m aware this was previously common in the housing boom, but the options of lenders seem to be significantly less.

3. Remortgage based on expected final value
It was my understanding that a re-mortgage can be secured on the expected final value, if presented correctly, i.e. with approved planning permission in place and firm quotes from contractors to complete all works.
However, a high street bank has recently informed me it would not consider this type of loan to anyone and only willing to loan on the existing valuation, either re-mortgage or advance.

4. Funding the extension with short term lending
If re-mortgaging on the existing value, there is a scenario that staying on a variable rate for the short term (3 or months) and funding the work with savings and personal loan effectively keeps borrowing costs down. Following the completed extension, we could apply for a re-mortgage on the new valuation and pay off the personal loan as quickly as possible. Any experience on this?

Thanks