Just 230 of the 4,000 Royal Institute of British Architects chartered practices have signed its 2030 Climate Challenge.
RIBA chief executive Alan Vallance urged firms to sign up to the "undeniably tough but progressively achievable" challenge.
"Changing the way we practice to combat climate change isn't an option anymore," he told Dezeen. "We need everyone on board to make a difference."
Initiative challenges studios aim for net-zero
This is in line with the decarbonisation goals of the 2015 Paris Agreement, which aims to limit global warming to 1.5 degrees Celsius above pre-industrial levels, as well as anticipated UK legislation.
Signatories must "attempt to meet the targets on all their new and major refurbishment projects and commit to submitting data on these projects."
RIBA told Dezeen that more practices had signed up recently amid growing awareness of the need to eliminate emissions from buildings, which contribute around 40 per cent of all greenhouse gas emissions.
"We know lots of practices are contemplating and reviewing the targets, which is really encouraging," said RIBA senior media officer Abigail Chiswell-White. "We just need them to sign up!"
Majority of UK studios not signed up
But the failure of the vast majority of practices to join the initiative follows concern from United Nations climate champion Nigel Topping over the lack of climate action by the profession.
Last month Topping told Dezeen that architecture "is one of the least well-represented businesses in the Race to Zero," referring to the UN initiative to get companies to commit to net-zero emissions by 2050.
Net-zero means that there are no net emissions of carbon across the entire lifecycle. For buildings, this covers both embodied carbon emitted by the construction supply chain and operational carbon emitted during the building's use. Unavoidable emissions must be offset using schemes that remove carbon from the atmosphere.
Emissions from the built environment are under the spotlight in the run-up to the UN's COP26 climate conference in Glasgow this November, where a day will be dedicated to the sector for the first time.
Topping said architects are in a "unique and important position" to reduce greenhouse gas emissions due to the influence they have on choices of materials, systems and construction methods.
He said his team was working to get architects to join the Race to Zero commit to net-zero "so that when we reach COP26 we can really show ambition within the sector."
Foster + Partners says joining scheme would be "confusing"
So far, none of the world's 50 largest architecture firms has joined the Race to Zero. Of the 20 largest practices in the UK, only 12 have signed up to the RIBA initiative.
Dezeen contacted all eight practices that have failed to join the 2030 Climate Challenge.
"We feel it would be confusing for our teams to have two similar systems running in parallel," the practice said, claiming that its manifesto covers "covers much of the same ground" as the RIBA challenge.
However, Foster's manifesto does not commit the practice to achieving net-zero emissions in its projects or set out how it can do this.
Zaha Hadid Architects uncertain it's "something we believe in"
"We are addressing these targets," said Zaha Hadid Architects, the UK's third-biggest practice and another that has not signed up to RIBA's 2030 Climate Challenge. "We are currently measuring our portfolio with respect to these targets and establishing robust systems to monitor the progress of every project."
But it added: "We would not make any public commitment without being certain it's not only something we believe in but also something we are able to deliver."
Sheppard Robson, the sixth-biggest UK practice, said it was about to sign up to the latest version of the challenge, which was updated recently. "Sheppard Robson are signing up to version 2 and are currently aligning our reporting to enable data to be shared with RIBA," the practice said.
"We were the first major architect practice to pledge to design and deliver socially and environmentally regenerative buildings and assets by 2030 and, as a first step to reach this, to design net zero-carbon ready buildings and infrastructure by 2025," it said.
However, the net-zero carbon statement on Grimshaw's website only commits the company to making its own buildings net-zero, not those it designs for clients.
Large studios now considering signing up
Stride Treglown, another top-20 UK practice that has not signed up, said the aspirations of the RIBA Climate Challenge 2030 "align well with our own ambitions and targets". However, it added that "the collection of data required to demonstrate performance against the RIBA's targets was a considerable challenge."
It said that version 2 of the challenge "has since made data collection more achievable and we therefore expect to add our signature in the near future."
EPR Architects said: "Our dedicated in-house sustainability team are currently reviewing the RIBA Climate Challenge data submission requirements, with the view to sign up shortly.”
PRP said: We have not signed up to the RIBA 2030 Climate Challenge as of yet as we are currently undertaking work to establish whether can exceed the targets. If we can, and we believe we will be able to, we want to make a statement that we are going beyond the targets set by the 2030 Climate Challenge."
Squire and Partners did not respond to Dezeen's questions asking why it has not signed up to the challenge.
Targets "undeniably tough"
The twelve practices in the UK top 20 that have signed up to the initiative are BDP, Allford Hall Monaghan Morris, Allies and Morrison, Hawkins\Brown, Atkins, Scott Brownrigg, Feilden Clegg Bradley Studios, TP Bennett, Purcell, AHR, WilkinsonEyre and PLP Architecture.
"The targets we've set our chartered practices through this challenge are undeniably tough, but they're also progressively achievable," said RIBA's Vallance. "We're here to provide our members with the guidance they need to work towards them."
"Changing the way we practice to combat climate change isn't an option anymore, and we're confident that these goals will place projects on the trajectory towards net-zero," he added.
In October, RIBA and climate network Architects Declare will host a conference that aims to hone a message to governments ahead of the COP26 conference the following month. The Built Environment Summit will generate proposals for regulations to help decarbonise the built environment, which it will present at COP26.
"We're seeing more and more practices sign-up to attempt to achieve these reductions on their projects ahead of COP26, but we need everyone on board to make a difference," said Vallance.
"The challenge is about committing to design for outcome-based building performance based on a clear set of RIBA standards – and I cannot emphasise the importance enough.”