Housing charity Threshold has raised concerns about the "potential unintended consequences" of proposed changes to exempted development regulations, including garden homes.
It follows a proposal by the Department of Housing, Local Government and Heritage to increase the allowance for such structures - up to a size of 45sq/m to the rear of a dwelling within its private boundary.
The changes would no longer require planning permission for certain developments.
Public consultation on a proposal ended two weeks ago and the department said it received over 900 submissions.
It said: "Over half of submissions by the public, of which are predominantly supportive, relate to the proposal to exempt modular-style homes up to 45sq/m to the rear of the dwelling."
However, Threshold warned of the risk of these developments entering the private rental sector market without robust safeguards or planning standards, as well as the elevated risk of abuse of older people in such arrangements.
It called for a rights-based and comprehensive regulatory framework that protects tenants, supports vulnerable older people and maintains national housing quality to mitigate against any unintended consequences.
In a statement, it said that while Minister for Housing James Browne noted the intention was not to expand the private rental market through the new measures, Threshold believes many of these units could be rented without the usual planning or safety frameworks, or tenancy protections, in place.
The housing charity said safeguards would be needed to protect renters, particularly more vulnerable renters, from living in potentially sub-standard and unregulated housing.
It called for oversight by planners of garden home developments.
Minister of State for Planning and Local Government John Cummins said responses from the public consultation will "help shape our proposals", including plans for modular-style homes at the rear of dwellings.
"I have always said this measure will only be appropriate in certain settings and with conditions that will be outlined in the draft regulations," he said.
Minister Cummins said regulations "have not been substantially updated in 24 years", adding "the close of the public consultation phase marks real progress towards realising fit for purpose Exempted Development Regulations".
"This will reduce unnecessary planning applications, free up local authority resources for larger-scale developments while also allowing homeowners to proceed with small-scale projects more quickly," he added.
