mullet wrote:..... Otherwise, your computation is broadly correct.
I’m not so sure as there are some clear errors in OP’s draft calculation (e.g. no last 36 months relief, PPR relief on annual exempt amount of £10,900).
If OP sell all of the house in March correcting per his draft calculation :
Total period owned for CGT (March 82-March 2012) =360 months
Main residence about 16.5 plus last three years = 198 +36= about 234 months
Cost = 37+2+5 = £44k
Gain = 210 sale -44costs = 166
Unrelieved gain = (360-234)/361 about 35%* 166= 58
Taxable gain = 58 -10.6 annual exempt amount = about £47.4k (rather than your estimate of £69.6k)
As Mullet comments, presumably OP has documentation to show that £210k is a reasonable estimate of “market value”, has never let out the house and elected new house as main residence from Sept 1998.
Isn’t one obvious approach to CGT mitigation is to (contract to) sell her only 50% this tax year ending 5/4/12 and after that date a separate contract to sell remaining 50% next tax year so you get an annual exempt amount against each partial sale?; assumes no other capital gains.