The Government has tabled an amendment to the bill that would, in "special circumstances", give charities exemption from or a reduction in the proposed community infrastructure levy, which will tax the value of land once planning permission has been given.
The measure will affect organisations that are building new premises or extensions to existing buildings.
Charity campaigners against the tax said the amendment was insufficient because they wanted a complete exemption from the levy, which is due to be discussed this Thursday in the House of Lords.
Helen Donoghue, chief executive of the Charity Tax Group, said the wording of the exemption was "not very reassuring". The group has led a coalition of campaigners to lobby for an exemption for charities from the tax (3 September, page 1).
"We're having further discussions with the Communities and Local Government department to see if they can go any way to meet our concerns," she said.
Nick Ivey, head of property law at solicitors Bates Wells & Braithwaite, said: "The amendment does not explain which charities will get exemptions, what level of exemption they will receive and in which circumstances. We cannot allow those things to be decided after the bill has become law.
"We would like to see a blanket exemption on the face of the bill. And we would like that exemption to apply not only to charities, but to all not-for-profit organisations."
The community infrastructure levy was introduced as part of the planning bill, which had its first reading in November last year.
It replaces an earlier tax proposal called the planning gain supplement, which was first suggested in late 2005.
It will be charged by local authorities on most types of new developments, depending on the size and type of project.