Buying next door house and knocking into one with ours

[Mama38]

Rather a dull tax question but .... I've read previous threads on this subject in general but does anyone know about Capital Gains implications when it comes to the following: knocking next door house into one with ours, during conversion section off upstairs area of second house to rent out and extend ours downstairs by knocking into one ... But when kids are grown - convert back to two houses and sell off the properties as two properties. Would the house be regarded as a 'second home' and therefore would we have to pay capital gains on any increase in price? Or as it has been our 'prime residence' for a while would we avoid it????
Trying to work out whether it would be financially worth it ....

[TheArticFunky]

Most of the time it isn't financially viable. My house and my neighbours house have a combined value of £700k, if they were made into one house it would be worth approx £500k and I reckon it would cost close to £100k to make the conversion. No idea about capital gains tax.