Urban development plans are…

Urban development plans are already in place to intensify current residential zones. We don't need to use greenbelt lands. This is not going to reduce the housing crisis.
UN reports already outline that many species in the greenbelts are at risk.

The greenbelt should be a belt of land without disruption as much as possible to support this ecosystem.
Housing development would further these risks and reduce the effectiveness of the greenbelt system. This would result in further flooding and loss of natural cooling effects from the trees.
The greenbelt areas are already under protected with allowances for highways and quarries.