But employers are disputing that top officers come under European rules, although they agree the 'long hours culture' in councils needs addressing.
Trade ministers last week released the long-awaited draft for implementing the European Working Time Directive in the UK, which gives most workers a maximum 48 hour week and guarantees three weeks paid annual leave.
The UK draft retains a derogation in the EU rules exempting 'managing executives or other persons with autonomous decision-making powers'.
But he welcomed the regulations' flexibility in allowing those who wanted to work longer hours to do so, with a duty on employers to monitor all employees' working time.
The regulations made existing legal responsibilities to limit working time clearer, Mr McGrath said. 'Our members are already being very flexible. We want the employers to take on board their duties.'
An MPO survey showed senior officers worked on average more than 52 hours a week and many felt unable to take holidays they were entitled to, he said.
The union has been invited to contribute to a Local Government Management Board study of the long hours culture in councils and would use the research as a basis, Mr McGrath said.
The Joint Negotiating Committee for senior officers is reviewing terms and conditions, with working hours one of the top items on the agenda.
JNC employers secretary Charles Nolda said councils disagreed that the working time regulations covered senior officers, 'but there is a problem about the long hours culture, in particular in some authorities'.
The LGMB was looking at the Directive's implication for different work groups throughout councils, he said.
Unison has broadly welcomed the regulations, but general secretary Rodney Bickerstaffe expressed concern at the right to individually opt out, saying variations should be by collective agreement.