Tollcross International Swimming Centre presently has an Olympic sized, 10-lane pool.
Work is under way to build a six-lane, 50m warm-up pool which will be needed by elite athletes in advance of competitions in 2014.
There will also be an additional 1000 seats for spectators and a first-floor concourse which will allow for a further 1000 temporary seats.
A further 2000 seats were to be housed in a temporary enclosure at the north of the existing competition pool to meet demand from people who want to see the world's top swimmers.
It had been planned to demolish this part after the Games which would have resulted in the swimming pool being closed to the public from April to December 2014.
A report to councillors said: "This will have a significant impact on every level of swimming programmes, from under-five classes to elite training and may impact on the perception of the legacy benefit associated with the Commonwealth Games."
And building a permanent extension would mean the pool would not have to be shut to the public for so long. Work is expected to start in March next year and the pool would reopen in May.
The venue would not need to close again until May 2014 when it will be handed over to Glasgow 2014 in preparation for the Games. It will reopen to the public in October 2014.
Archie Graham, Glasgow City Council's executive member for the Commonwealth Games, said: "The refurbished centre will be a fantastic facility for the people of Glasgow."