Morris – if the Owners Corporation (O/C) of your Plan wants to handle the sale / purchase properly, so that the Owner involved bears the costs of acquiring the additional land portion, has that shown on their Property Title instead of on the O/C’s Title, and has the Unit Entitlement for their Lot adjusted to reflect the resultant increase in value due to the extra land and the extension, then the process (in NSW) is:
- All Owners need to agree to the sale and to the proposed extension via resolutions at a General Meeting, which I believe needs to be unanimous for the former, so it might as well incorporate the consent for the extension as an “addition to the common property”.
- A Valuer needs to determine the value of the acquired land and of the extended Lot as proposed, and the Unit Entitlement for the latter.
- A Surveyor needs to prepare a Plan of Strata Sub Division showing the reduction to the Common Property and the extent of new (larger) Lot.
- The Valuer’s Report and the Surveyor’s Plan need to be submitted to Land & Property Information (incorporating the former Land Titles Office) for Registration of the changes to the Common Property and to the Owner’s Lot.
The Owner concerned would pay the O/C for the extra land based on the valuation, for the costs of the valuation, the survey/plan, and for the Registration/s.
Of course your O/C needs to ensure that full access to the pump area is formalised.
The Owner probably won’t wish to proceed now, but if they do there’s a bit more to be done with the inclusions in the O/C’s consent to the extension, but that may best be covered later 
.