Would really like to know the definition of extending existing plumbing?
I beleive in most strata plans, not only the plumbing but the tiles are considered common property. The problem is, people want to be able to renovate however they like and have someone else pay for the consequences. So, they want to replace the tiles made in China with the Italian marble tiles. But when something goes wrong, they want someone else to pay to replace the marble tiles that they did not get permission for when they did their bathroom renovation.
This is exactly why we had a by law drawn up that made the tiles and the maintenance of bathroom plumbing the responsibility of the owners. Owners wanted to be able to renovate to their hearts content, then have the OC pay for replacement/repairs if it all goes awry. Now they can put down what ever tiles they want and they will forever be responsible for it, but they still can’t change where the plumbing is or move walls etc maintaining the structural integrity of the units (we hope) without seeking approval.
If there was nothing wrong with your bathrooms, and you just wanted to improve, then it is a renovation and really to your benefit so you should be responsible. If it was a repair, then the OC would be responsible but I believe they only have to replace like with like so if you have 100 x 100 lime green tiles, they only have to find similar and then they would be responsible for ever. If you wanted a change with the repair ie 200 x 200 then they may be considered a renovation.
A nice bathroom really does sell a place. So if it was me, I would apologise profusely, and agree to the terms (by-law) and be pleased to have the place you want to live in and that someone else will appreciate in the future.