Calculating base cost, absence of paperwork

Given the statutory retention period for records has long passed on a property with enhancement in 1999 I'd find very few people able to produce such documentation at this point ( although there always a few who keep absolutely everything ). Revenue can ask you to produce evidence in support of your computation but at the same time will have permitted you to have disposed of records that old.

What's the basis for the amount of €Y which was spent on enhancement? Best recollection? Some scribbled notes which were kept?
Is there any pictorial evidence of the before and after? Was there maybe a plan or drawings kept ? Did the work require planning permission which might still be on record at local authority? Was it domestic or commercial, if commercial would the alteration have resulted in a change to rates? Are there other people who can attest to the fact that the work was done? Any of these would help support the justification for inclusion of the enhancement expenditure.

If a reasonable effort can be shown to be taken to quantify and evidence the enhancement then if the return is audited the taxpayer should be able to show that they acted reasonably and honestly.

Alternatively, only include everything which is clearly provable and pay the ~€5K CGT.