The calculation when you eventually sell the house will be done on a time apportionment basis - so if for example you own the house in total for 20 years, and you let it out for 8 years, 8/20 of the sale proceeds after expenses less the original cost will be chargeable to tax. However, you can always claim the last 36 months of ownership as exempt, so in this example the gain would drop to 5/20. In addition, there is something called lettings relief, which allows for further exemption of the lower of �40000 or the proportion of the gain relating to the time you lived there. This is a very valuable relief. In addition, as mentioned above, there will be taper relief based on the time you own the house before you sell it - this could be as much as 40% of any remaining gain after the above reliefs. Finally, you have an annual exemption (curently �8800). The cost of any capital improvements to the property Inot maintenance) will also be allowabe. You need to remember though that any of these reliefs and exemptions could change in future budgets.