CGT on developed housing

[Rich172]

looking at a few options.....
scenario:
2x barns owned as part of main residence.
150K conversion costs to two residences
both residences 50:50 joint tenancy with unmarried partner
each residence valued at 200K at date X.
properties empty for several months, letting commences date Y
value on date Y 210K each
Sold after 10 yrs @ 300K each

what is the starting point for CGT calculation?
is it 200K each (when they become stand alone properties) or do we go right back to the 150K conversion costs....

I assume the date Y value of 210K is the limiting factor vs allowable mortgage limit offset vs rental income (value of property when letting commences)

many thanks

[Telometer]

150k, plus a proportion of the original cost of your original main residence. Have you included architects fees, etc. etc. etc.?

As to £200, or £210k, you can probaby argue it either way. When did you first intend to let them out - as opposed to selling?