Shed to Home Office ?

Garden has a shed for over 30 years - in past was used as workshop but not for past 20 years. Its approx 29 sq m. and in a street of terraced houses in Dublin
Would like to convert into home office now and I know that without planning permission it should be 25 sq m. As it stands there is still enough of the garden left.

My question is as the shed is there for so long can I just convert to home office as is or should I reduce size to permitted 25 sq m. ?

Enforcement isn't an option on something that has been there for so long, but once you go about redeveloping it, the regulations and limits apply. It would be best stick within the exempted development guidelines.