adding value and CGT ?

I have an IP that I have for several years ( post 1985). It does have a large CGT due, if sold. I am thinking of adding value for sale. I’m hoping that the profit will cover all the CGT plus a bit more. To work out the extra CGT on the reno. Is this calculated on the sale, minus the value before reno, minus the cost of reno?
To get the 50% discount does this need to be over a calendar year of a financial year?
Also is the time from when work started or from when completed?

Can you recommend a firm /person who could do some projections on the possible cost / resale value?
Thanks :)