The move would prevent retailers like Asda from sweating assets by building mezzanine floors in stores where they cannot extend externally.
MPs attended a parliamentary briefing on Tuesday with pressure group Friends of the Earth, which has been lobbying parliament for months to make internal and external extensions subject to the same restrictions.
Lib-Dem MP Matthew Green said he was trying to set up a meeting with planning minister Keith Hill to discuss the options, but expected an amendment would be tabled in September before the Bill went back to committee stage in October. Assuming there are no major glitches, the Bill should become law by March 2004.
FoE executive director Tony Juniper said: "Large scale mezzanine floor or basement extensions, like new stores, can have very significant impacts on local communities and businesses.
"The scope for uncontrolled expansion is considerable and could make a mockery of national planning guidance which seeks to protect town centre shops, local communities and the local environment."
Asda, which is building mezzanine floors at stores in Sheffield, Cumbernauld and Govan this year, declined to comment before details of the amendment had been released.