The new firm will have a 145-strong staff and will open a new office in Bristol within the next few months.
Devereux director Nic Allen said: 'This exciting move will create a significant pool of resources, enabling us to take on larger education, healthcare and other sector projects in a wider geographic area.'
DevereuxLHC's corporate structure will form an 'umbrella' limited liability partnership that will run alongside the existing practices of both Devereux and LHC. These will operate from their three existing offices in addition to the new Bristol operation.
LHC director Peter Lacey said: 'The two practices are a natural fit and we welcome this opportunity to gear up a few sizes and take our education and regeneration experience to a wider market.
'This step has enabled us to fast-track to a point that would have taken many years to reach by organic growth alone,' he added.