Hello Alex, i am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
Calculate the cost of the building at market value as at building date, add it to 35K and the difference between that and 150K is your capital gain. Deduct your non cumulative Annual Exempt Amount (AEA) of 11.1K leaves a sum exposed to Capital Gains Tax (CGT) at 10% or 20% or a combination of the two rates depending on your income including the gain in the tax year of disposal.
I do hope that you have found my reply of assistance.
With respect you are a builder so you should be able to estimate that cost and, yes, materials are included.
Yes, that would be appropriate.
This is most unlikely and would only occur were HMRC were to make an investigation, which is not much of a possibility these days. You would then show them the figures based on average industry prices. You should keep a record of how you calculated these to maintain with your materials bills.
Thank you for your support.