Hello, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
The trite answer is as long as the proverbial piece of string!. However, take the purchase price add the purchase costs including Stamp Duty Land Tax then add the costs of doing up the premises. This will give you an acquisition price. Now note the net selling price ie after deduction of selling costs including advertising and take away the acquisition price. The figure with which you are left will be the gain for Capital Gains Tax (CGT). Now deduct your non cumulative Annual Exempt Amount (AEA) of 11.1K and the remaining figure will be the gain exposed to CGT. This will be levied at 18% or 28% or a combination of the two rates depending on your income including the gain in the tax year of sale.
There is one danger. HMRC may class this activity as trading in land and make the gain subject to Income Tax (IT) as opposed to CGT. That Department has been known in the past to classify just one such transaction as so trading.
I do hope that you have found my reply of some assistance.