The warning came as countryside campaigners prepared to lobby Parliament against housing developments on green-field sites. John Prescott, the Deputy Prime Minister, has already given the go-ahead to Green Belt building schemes including one for 10,000 homes near Stevenage, Hertfordshire.
But the House of Commons Environment Committee heard from academics that grand schemes to meet projected demand could be misguided.
Planning guidance from the last government had already provided for up to 1.1 million more sites than were needed, said Professor Alan Wenban- Smith, formerly a local authority planner and now an economic development consultant. Recent upward revisions of projected housing need could add another million to this figure. The surplus would mean developers would cherry-pick green-field sites and would avoid more environmentally desirable but also more risky building projects on reclaimed land in cities.
Another academic, Professor Glen Bramley of the School of Planning and Housing at Edinburgh College of Art, said national figures on how many houses were needed would not be sufficiently accurate at local level. Predictions were made by breaking down census data into family types, age groups and other categories and once this had been done within a single local authority the numbers might be insufficient to make accurate predictions.
He also suggested national figures might not turn out to be accurate. "I would probably expect the out-turn to be somewhat below the 4.4 million at the end of this period, but then I would almost certainly be wrong, as everybody else has been in the past," he said.