Extension And New Build On Same Plot

[Tombo1]

I am hoping someone can give me some advice.
I have a two bedroom cottage on a large corner plot upon which I recently, successfully, applied for planning permission to build a new four bedroom house at the bottom of the garden.
My question is that the original house is simple to extend. Probably to a five bedroom. I would like to do this. I would like to enjoy the larger house whilst we can. Having family who live abroad come to stay etc. So if I extend the cottage, enjoy the property until we feel the need to downsize and then build the new house which we would move into long term,(my wife is partly disabled so it will be more suitable when a little older), what is the tax liability on the improvements/ extensions/ New Build assuming I sell the extended cottage to move into the New Build?
I thank you all in advance and look forward to any suggestions that might mitigate any tax liability if there is one presently.

[pawncob]

As it will be your PPR at all times, no CGT arises on the sale.

[Tombo1]

Thank you for the reply.
If I carry out the extension and new build then rent out the extended cottage and move into the new build, lets say for three years, then move from the new build back into the cottage and sell the new build. What liability would be due then?

[pawncob]

The new build would be your PPR, so exempt. (but part of extended would be chargeable)