No arguement intended, I am suggesting that the easiest way to put value on the property is to get planning permission for an extension.
You do not need to follow through with the building of said extension as the budget will not allow for this as you have stated.But you would have to go through your LABC.
You said any prospective buyer would hopefully see the potential,if you have done the above,the property will be marketed with full planning permission,therefore unmissable.
I really hope you understand what I am trying to explain to you, either way no harm implied and good luck with whatever route you take.