ACT Regulatory Services Minister Gordon Ramsay said from next year, mandatory tests may be required for builders across the A, B and C classes to renew their licences.

One in five builders will be made to sit the exam. However builders who have had a complaint against them substantiated, or who have been subject to disciplinary action in the past are more likely to be targeted.

“Our message to industry is clear: lift your game or do not build in the ACT," Mr Ramsay said.

Access Canberra deputy director-general Dave Peffer said a single substantiated breach would be enough to force a builder to sit an exam.

"We recognise there are plenty of good builders out there, doing the right thing, not cutting corners and building great buildings in the territory but equally so there's a few out there doing the wrong thing," Mr Peffer said.

"These examinations will be targeted at those individuals at the point of renewal to try and bring up the overall standard of the industry."

Mr Peffer said Access Canberra wrote to 274 builders putting them on notice that they had substantiated complaints about quality against them, and they would be targeted for testing.

He said complaints would have to have been investigated by the regulator to count.

"We investigate the complaints as we receive them, sometimes that can be a short process, other times it's quite an extensive process that requires engineering reports.

"Once we've substantiated there is a legitimate complaint around building quality they add it to the list."

All builders applying for their A, B or C class licence for the first time will be forced to sit an exam from later this year.

If a builder fails the test twice, they will not be allowed to apply again for a set period yet to be determined- regardless of whether they are renewing their licence or applying for it for the first time.

In 2016, 48 per cent of applicants failed the Class C exam the first time, and 44 per cent failed on their second attempt.

ACT Planning Minister Mick Gentleman also said there would be changes over the next 18 months to improve the quality of apartment buildings.

This would include introducing guidelines for for people preparing building approval applications for complex buildings, including apartment buildings; new codes of practice for builders and certifiers; and a new auditing system for building approvals and projects.

Shoddy building work has cost Canberrans more than $100 million in the last year, with horror stories about home owners left in the lurch after broke builders walked away from half-finished, poorly-built projects, and body corporates reluctant to go public about leaky apartment blocks for fear of devaluing members' properties common across the city.