The Kilmartin Museum in Argyll, near the site of Dunadd hill fort, seat of the first recorded kingdom in Scotland, is to be transformed, with new museum and gallery spaces, an education centre, a café and expanded storage areas.
The commission follows the Edinburgh firm's success in securing the contract for the new Bannockburn visitor centre, a £9.1m project funded by the National Trust for Scotland in partnership with Historic Scotland, due to open next year. Acclaimed recent commissions include the Pier Arts Centre in Orkney and Forth Valley College in Stirling.
Dr Sharon Webb, director of the Kilmartin Museum, said: "We are a victim of our own success - our archaeological collections have grown hugely over the last 10 years and we need more space to care for them and ensure people can enjoy and understand them."
David Anderson, senior associate at Reiach and Hall, said: "What makes the job interesting is working with passionate clients who care very deeply about what they do.
"Neolithic Orkney is very well known, but Kilmartin, where pieces of pottery have been found that pre-date anything found at Stonehenge, is now coming into the spotlight."
Kilmartin Glen in mid-Argyll, 90 miles west of Glasgow, contains more than 800 historic and prehistoric monuments, cairns, standing stones, stone circles and rock art, some dating back 5000 years.