Part of the Heart of Slough regeneration scheme, the current 1930s town hall will be redeveloped, rehousing council staff in a renovated building with significant new additions.
The firm was originally running against five practices - with the shortlist also including HOK International and Sheppard Robson - but the two practices dropped out of the race.
According to HLM, the town-hall project will include updating the working conditions for staff and providing the civic headquarters with state of the art technology, and will also deliver a substantial new building element to the site.
The 12,500m2 scheme is a significant win for HLM, which is steadily making a name for itself in the local-authority sector.
The practice is also undertaking a great deal of work for the London Borough of Greenwich, providing new civic headquarters as well as a library and local service centres.