Hi,
Only those expenses that fall into the category of 'capital expenditure' can be offset against Capital Gains. Examples of capital expenses thast would normally be allowable are (a) adding an extension (b) installing a security system id there wasn't one before and (c) replacing a kitchen with one of a higher specification.
Detailed guidance on what qualifies as capital expenditure is provided at:
CG15150
CG15292
CG15150P
Thank you.