But a recent consultation between April and July on its proposals showed that only 20% of respondents agreed with the proposed threshold and prompted Defra to review whether this is the best figure to use.
Speaking at a Materials Recycling Week conference in London on Tuesday, Defra policy adviser Julie Faria said the department was commissioning research to look at the costs and benefits of construction activity.
"We are looking further at the costs and benefits and that will help to inform the threshold level," she said. "We will need to look at the research and then consult on its findings."
She added that Defra was also considering whether project value is the best way of assessing a scheme's need for an SWMP.
Faria said that there was a possibility that further consultation on a new threshold level could push the introduction of compulsory SWMPs back from April 2008, although some time had been allowed on the current implementation timetable for further investigation of issues raised by this summer's consultation.
Currently, 10% of construction waste is unused materials and 26% is packaging.
An SWMP comprises a checklist of steps that contractors should take when planning waste disposal, with the aim of reducing waste through detailed planning.
Research commissioned by the Waste Resources Action Plan (WRAP) in early 2006 shows that only 11% of the top 800 construction companies use waste management plans and only 3% follow the DTI's 2004 guidelines for implementing SWMPs on a voluntary basis.