Do we need an easement?

First time poster here – apologies if the topic’s already been covered.

Our 1970s era apartment block has garages on the ground floor. We have original condition vertical sewer stack pipes that run vertically through these garages, then underground, before connecting to the main sewer lines on the common property.

We’re just initiating the process of replacing these original sewer pipes, and based on recommendations of a number of plumbers, we’re considering re-routing the pipes as they enter the garage so they run along the garage ceiling to the nearest exterior wall, and then down the outside of the wall. That would allow the addition of inspection points on the common property, and remove underground piping that’s in private garages (aka hard to access). Private owners would lose some space at garage height level, but would gain the space currently occupied by the vertical pipe.

Currently there are no easements defined for any of the lots. Our Strata Manager’s lawyers are recommending that we need easements defined (and I gather that will potentially also require a surveyor?)

Is this necessary? Can we get the owner’s permission in writing for the works, without an easement? Or is an easement necessary?

Thanks in advance,
AC