It could definitely push up the value of the house, but as has been pointed out there’s no guarantee you’d get it.
You’ve got a couple of options, go to an estate agent and tell them your thinking of moving and they’ll value your house. Ask them if they see potential for a plot and ask them for some figures, get a value on the house as it is, then the house with planning permission, then a value for the house minus the big chunk of garden and get a value for the plot. See how the numbers stacks up.
Some things to be mindful of, staying in the house and selling a chunk of your garden off may incur capital gains tax liabilities, selling your house and transferring the plot to yourself for you to build a new place on may not attract tax so it’s worth thinking about. You can’t be bothered with all this, what about putting a clause in any sale agreement to the effect that if the garden is developed you get a share of the proceeds?
Start with the values, if they are decent and you are thinking about it I’d defo go and speak to a solicitor and an accountant. Most will give you a free initial consultation. If it’s a runner, you may need to engage a planning specialist or an architect or even both.
You in Carmarthen?