As a starting point the value of the property today will indicate whether you are likely to have a CAT problem and as was mentioned, other gifts that you may have received will effectively reduce the 'threshold'. If that is even remotely likely, you should try and get the receipts for the improvements, if they or other than 'repairs'. I would have thought that you would have a case then to reduce the value for tax purposes.
If your mum is a dependent relative, and you borrowed the money for these improvements, the interest may also be relieved.
I think the Tax Office to be fair, will be helpful in advice on this, or you could discuss it with a tax professional as was mentioned.