Chief executive promises no radical change as 150-strong surveyor becomes consultant's 28th buy
WYG acquired the QS, which employs 150 staff in offices in London, Liverpool, Manchester, Birmingham, north Wales and the Isle of Man, for £11m.
About 75% of Tweeds' clients are in the private sector and 85% of the business is focused on building rather than engineering.
WYG has been quietly expanding for the past decade, making 28 acquisitions in that time.
Chief executive John Purvis said that in the "immediate" future at least, the Tweeds brand would remain. Tweeds is bigger than WYG's management services arm and the combined business will employ 250 people.
Peter Swift, director at Tweeds, has been appointed managing director of the enlarged management services division.
Purvis said WYG, which has more than 2200 employees, was in talks with Tweeds for about six months before the deal and that it had looked at other QS firms.
Tweeds had a strong client base among private developers
John Purvis, White Young Green
"We looked at the market but Tweeds had a very strong client base among private sector developers and provided a platform to grow our management business," he said.
Purvis said no fee earners would lose their jobs as a result of the acquisition because it was about "growing Tweeds, not dismantling it".
Tweeds will continue to operate from its existing offices and Purvis said "nothing radical" was planned in terms of changes at the company.
WYG announced the deal on Tuesday as it revealed a 15% rise in pre-tax profit to £5.3m for the six months to 31 December. Turnover rose 19% to £78.6m.
It has also bought Farningham McCreadie Partnership, a planning business with offices in Edinburgh and Belfast for £2.2m. It will be assimilated within WYG's planning business.
Purvis said no other purchases were imminent, but would not rule out further acquisitions this year.