Around one in four homes currently advertised for sale would be liable for a Home Information Pack (HIP) if they were being freshly marketed after August 1, figures show.
Earlier this week, Communities Secretary Ruth Kelly announced watered down HIP proposals that would require sellers of four or more-bedroomed houses to commission a pack before listing their property from that date.
Property website Rightmove said that at present around 25% of houses it lists fall into that category, although speculation is rife that incoming HIP regulation could see 'four bedroom houses' being marketed as 'three bedroom plus study' homes.
Of the more than 600,000 properties listed at present, 221,954 are classified as "large houses".
Miles Shipside, commercial director of Rightmove, said: "This is a larger proportion than has been speculated.
"It remains to be seen whether this percentage will be representative of what comes onto the market after August 1, especially if there is a rush of instructions prior to that date to avoid the costs of a HIP."
The property website registered a recent upturn in the total number of houses coming on the market in advance of the original HIP implementation date of June 1.
But with the phasing in of the scheme from August, Rightmove now expects a less dramatic effect on the number of new listings but warns that there could still be a surplus of larger properties.
Mr Shipside added: "I think there is a loophole that some will look to exploit. You have to have 'commissioned a pack' to market a 'four bedroom-plus' property after August 1, not physically have documents in your possession.
"This will be virtually impossible to police, so I can see lots of documents not being ordered 'til a purchaser has been found, rather than ordering and paying for them up front."
