The company is now located at in a former textiles factory on Great Sutton Street, in the Clerkenwell area of London.
Senior partner Charles McKerracher said the new premises signal the beginning of a major expansion in staff numbers, which currently stand at 80.
He said: "We're actively recruiting a number of posts within the London office to spearhead our expansion plans and deliver a number of new instructions we have received recently.
“This expansion is also mirrored in our Scottish offices, in both Edinburgh and Glasgow, where we are also seeking to recruit for a number of positions."
Doig+Smith, which was founded in 1936, is currently working on the £1.3 billion Heathrow East project to replace Terminal 2 and the Queen’s Building.