Every time I've remortgaged I've given my IFA an estimate for the increased value of the property and it's only ever been questioned once; and the mortgage company sent a valuer who ended up agreeing with my estimate anyway.
The mortgage company can see how much you bought it for and when, and how the prices have increased on similar properties or your area, so they'll likely take a new value if it's reasonable.
One thing to note is that the value a mortgage company assigns to a property isn't necessarily the same as the price that people are willing to pay for the things.
This is now on just across the road from us. Our is a 2bed maisonette, refurbed before we moved in, has GCH and we have our own garden (complete with hyper-awesome-mega-shed).
That's pretty much it.
Every time I've remortgaged I've given my IFA an estimate for the increased value of the property and it's only ever been questioned once; and the mortgage company sent a valuer who ended up agreeing with my estimate anyway.
The mortgage company can see how much you bought it for and when, and how the prices have increased on similar properties or your area, so they'll likely take a new value if it's reasonable.
One thing to note is that the value a mortgage company assigns to a property isn't necessarily the same as the price that people are willing to pay for the things.