OK. So lets assume that it cost £50K, he and your mum own it jointly. If the market value now (pre extension) is £150k, he and your mum would pay CGT on their gain when he sells it to you. £50k gain each, knock of the annual exemption (say £10k) £40k each taxed at 18% = 7.2k each. In tax to pay at 31 Jan 2011.
Then he buys back, presumably the market value hasn't gone up, then he goes extension, makes (say) £50k gain, and pays tax on that gain (say next year, due 31 Jan 2012)
This isn't really achieving much. It may divide the gain in to two tax years, saving £1800 by using two allowances, but the SDLT on both transfers cancels that out. If there is any intention not to pay SDLT, then whatever he hopes to achieve won't work. If £150k is not the real market value, then because you are connected, the transactions will be deemed to take place at market value.
If what he want to be able to do is not tell HMRC about the sale to you, and then claim the buy back as his base cost, this is fraud, and is likely to be picked up.
For this amount of tax, any scheme that worked would not be worth it in accountants and solicitors fees to check that it does. If you don't see advice from a solicitor or accountant explaining excatly why it works and putting numbers in, don't do it. If you do see one, check it out with us!