There has been a surge in under-occupation over the past few decades, driven largely by people living in family homes long after their children have moved out.
Between 2017 and 2022, the number of homeowners with at least two spare bedrooms jumped by 950,000 to 8.25 million, according to the English Housing Survey.
This means there were more than 16.5 million unused bedrooms in owner-occupied homes across England.
Stamp duty is paid on the purchase price of a property in England and Northern Ireland. It is charged in bands, starting at 5pc for the value of a property between £250,000 and £925,000, rising to 12pc for values above £1.5m.
Mr Bax said stamp duty is a particular disincentive for older people because their changing needs mean they might not be able to live in a new home for a long time, and they are trying to save money in case they need to pay for future care.
He said: “The psychology of the older person needs to be understood in that equation. They are giving money away that they could be using either to fund their health needs in the future, or passing on as legacy to the next generation.”
Because stamp duty is taxed in bands, house price rises have triggered disproportionate increases in stamp duty bills, meaning the tax has increasingly become a barrier to moving house.