Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
As the gain from the sale of the property can only be taxed in one tax year, you just need a figure for the labour costs from the builder. It would be helpful if he could provide some detail of the work done as HMRC may ask for proof of the expenditure. Are you not claiming for the cost of materials and fixtures and fittings?
The estate only qualifies for a CGT exemption for the tax year of death and each of the following two tax years. It will also pay CGT at 28%. If your Mother is a basic rate taxpayer, it might be worth putting the property into her name before it is sold so as to benefit from the 18% CGT rate on at least part of the taxable gain.
I hope this helps but let me know if you have any further questions.
I'm just following up to find out if my answer helped or if you have any further questions.