I don't know who has suggested this to you but I suggest you speak to a Chartered Surveyor who specialises in valuing first. It may be a good idea - but you really need to understand the parameters.
If, say, you have a small house on a large plot and it is smallest house on the road, then configuring it in the way you suggest may be beneficial. But you might get much of the planning gain merely by GETTING THE PLANNING PERMISSION (so that buyers can see what is possible).
If you have a house the same as others on the road and you've already extended it - then it is probably not a good idea.
As a principle, unless you are turning into a property developer, you should extend your house in line with what your needs are.
The simple truth may be that you are just asking too much money for it - sorry.
Procedure - talk to Planners to see if it fits the planning policy, get plans drawn up, get planning permission, (for a two-storey extension) get building regs drawn up - unecessary on a small extension, get builders to quote on basis of drawings, contract with builder, get work done, pay for work. You'll have to work out where 'ask lender for more money fits in- probably after the very 1st step.