The deal makes WYG one of the largest planning consultancies in the UK with around 100 planning specialists.
Alliance Planning turned over £2.7m last year, making a £539,000 pre-tax profit.
Established in 1996, the Guildford headquartered, business quickly developed a network offices in London, Birmingham and the East of England.
Clients including Frasers Property, Henry Boot, Persimmon Homes, Telereal Trillium and several housing associations, city and county councils.
The deal was announced as WYG also announced orders were up by 10% at the half-year to £95.5m.
Paul Hamer, chief executive officer of WYG, also confirmed the group would be seeking to resume dividend payments at its upcoming AGM
He said: “This acquisition, together with recently announced major project wins, represents a further significant step in WYG’s growth journey.”
“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.”