Terrace Hill, which has its registered office in Edinburgh plus another base in Glasgow, said the all share transaction for Urban&Civic would give it greater scope to fund larger developments.
The deal values Urban&Civic at more than £95m and has gained backing from shareholders holding 72.8% of Terrace Hill's share capital. It plans to issue more than 75.5 million shares at 225p each.
The proposal also involves Terrace Hill moving from the Alternative Investment Market to the main London market and being renamed as Urban&Civic.
London based Urban& Civic, formed in 2009 by property veterans Nigel Hugill and Robin Butler with backing from GI Partners, has a major site at Alconbury, around 20 miles from Cambridge, which has outline planning for around 5,000 house and has a gross development value of £1.3 billion. It also has a 50% stake in a site in Rugby, Warwickshire, with planning permission for 6,200 homes and 1.3 million square feet of commercial development.
Among Terrace Hill's portfolio is the construction of four restaurant facilities along the quayside at the Broomielaw in Glasgow.
Mr Adair, executive chairman at Terrace Hill and holder of a near 63% stake, will become non-executive deputy chairman at the enlarged group and subscribe for £640,000 of new shares as part of the issue.
A general meeting to vote on the deal will be held on May 14. If approved Mr Hugill will be executive chairman with Robin Butler managing director. Terrace Hill chief executive Philip Leech will be property director while the company's finance director Jon Austen will retain that role in the bigger entity.
Mr Adair said: "This powerful new combination represents a transformational and exciting step forward for us. I could not have wanted for a better and more promising outcome."
Shares were up 1.38p, or 6.2%, at 22.12p.