Acquisition of 32-strong Ruddle Wilkinson will increase Capita staff to 200-plus, making it second only to BDP
After the deal, which is expected to go through on Monday, only Building Design Partnership will employ more chartered architects.
It is not yet clear whether Ruddle Wilkinson, which employs 32 chartered architects at offices in London and Peterborough, will keep its name or be merged with Capita Percy Thomas.
However, it is understood that Rob Firth, who already heads up Capita Percy Thomas, will continue to manage the expanded business. The future roles of managing director Norman Taylor and chairman John Durance, are as yet unknown.
Ruddle Wilkinson's integration into Capita Percy Thomas will create a practice of more than 200 chartered architects.
Ruddle Wilkinson specialises in transport hubs such as airports and railway stations as well as education, office, retail and residential schemes.
The firm left a massive refurbishment of London Tube stations just before Christmas, but has worked on parts of Heathrow Terminal 3 and expanded Keflavik Airport in Reykjavik, Iceland.
It is also the firm behind the first city-centre Ikea store, due to open in Coventry next summer.
Capita Symonds declined to comment on the deal, which is set to be the first of a series of planned acquisitions over the summer.
Managing director Richard Marchant last week outlined plans to increase the firm's current £200m turnover 50% in the next three years.
Further acquisitions are expected to focus on setting up a presence in Birmingham, Leeds and Newcastle.
Recent purchases have included Birmingham architect Church Lukas earlier this year, and architect Norman + Dawbarn and transport consultancy Buchanan Consulting engineers last year.
Marchant said last week that companies in the frame for acquisition had to be "the right business with the right returns and the right culture as well".
Mike Nightingale, whose Nightingale Associates firm has now been leapfrogged by Capita Percy Thomas, cautioned that expansion had to be about quality not quantity.
He said: "Size isn't everything. Unless you have a specific specialism it's sometimes the case that the bigger you get, the more difficult it is to manage. Getting big for the sake of it just causes management headaches. If you are going to get as big as that, then the jury is out on the quality of the work."
Ruddle Wilkinson was unavailable for comment.