WYG said that the deal created one of the largest planning businesses in the UK.
WYG has also issued an upbeat trading update, with a 10% increase in orders and a resumption of dividends for shareholders.
Under the ownership of its directors, Alliance Planning turned over £2.7m last year and made more than £500,000 in pre-tax profits. Its clients include Frasers Property, Henry Boot, Persimmon Homes, Telereal Trillium and a number of housing associations, city and county councils.
Since the end of its financial year on 31 March 2014, WYG’s order book has grown from £86.8m to £95.5m. Significant wins in the half year include the award of a place on the Ministry of Defence's four-year principal support provider framework for services to the Defence Infrastructure Organisation and, in a consortium with Denmark’s Cowi, a three-year contract with the European Investment Bank to provide technical assistance to the Infrastructure Projects Facility in the Western Balkans.
Chief executive Paul Hamer said: "We are delighted to be joined by Steve Fidgett, Gerald Sweeney, Andy Stallan and the team from Alliance Planning. We believe that they will be an excellent fit with WYG, significantly boosting our planning and asset creation capability. With approximately 100 planning specialists, WYG now has one of the largest teams in the UK. We are also increasingly able to use these specialist skills in support of our international business as they help to develop infrastructure related to population expansion and urbanisation.
"This acquisition, together with recently announced major project wins, represents a further significant step in WYG's growth journey."
He added: "We have enjoyed a very positive start to the year. In the UK, we have retained or won the overwhelming majority of the key framework agreements that we have bid for, which will sustain a substantial proportion of our expected activity over the next 2 to 3 years. We have also won a number of important new international contracts, significantly improved our order book and pipeline, and further strengthened our business through acquisition and other investments.
"In recognition of the continued momentum and positive outlook, we will be seeking shareholder approval at our upcoming AGM to resume dividend payments, which reiterates our confidence in WYG's financial position, prospects and profitability."