Her comments are at odds with the current local development plan (LDP) system in Wales, which does not require councils to work on a cross-boundary basis.
Hart’s apparent intervention comes after Cardiff City Council leader Rod Burman earlier this year strongly criticised the LDP regime.
Speaking to Planning in July, Burman claimed that the LDP system does not allow for housing numbers to be planned across a functional economic area.
He said: "There is no mechanism for agreeing the needs of the wider region between different local authorities. LDPs are judged in isolation."
Local development plans were introduced in 2004 and all of Wales' 25 planning authorities are required to produce one.
Cardiff's first attempt at an LDP was submitted to the Welsh Government in November 2009. However, it was dropped after planning inspectors raised concerns relating to flood risk, waste, transport and housing.