On his return to Edinburgh in 1927 Lindsay was articled to Reginald Fairlie and struck up a friendship with James Smith Richardson, the principal inspector of Ancient Monuments, travelling with him whenever circumstances allowed and visiting Norway, Sweden, Denmark, Italy, France, Germany, Holland, Belgium and Switzerland. He commenced practice on his own account at 5 Castle Street in 1931, quickly becoming prominent as editor of the RIAS Quarterly. He was early appointed to the Council of the Cockburn Association, and through Fairlie, became a member of the Friends of Falkland, an association formed by the Marquess of Bute and his nephew Major Michael Crichton-Stuart, later to become an influential figure in the National Trust for Scotland. On 7 January 1932 at Stoke-by-Clare, Suffolk Lindsay married the Hon Maysie Elizabeth Loch, daughter of Major General the 2nd Baron Loch of Drylaw and Stoke College, Compton Wynyates being loaned to them for their honeymoon; subsequently they motored through France in a small and even then antique Renault which still exists at Myreton Motor Museum. Although never sold, it was subsequently replaced by a Rolls Royce 'Doctor's Coupe' for business use and a large Bentley saloon for family travel, a fleet of vehicles subsequently supplemented by his father's Rolls Royce. On their marriage the Lindsays set up house at 56 Castle Street where they remained until 1938 when they took a house at 9 Inverleith Row, quickly followed by a further move to Houston House, Uphall, where the coachhouses were able to accommodate their collection of veteran and vintage cars. At these houses they built up a wide circle of professional friends, particularly notable amongst them being the liturgical historian and secretary of the Council for the Care of Churches, Dr Francis Carolus Eeles. Lindsay's sister Ailsa married ____Findlay, son of the architect Lt Col James Leslie Findlay and grandson of James Ritchie Findlay of 'The Scotsman', bringing a still wider range of influential contacts.

In 1933 Lindsay replaced Bryce as partner in the firm of Orphoot Whiting & Bryce. This firm had a somewhat complex history and very good social connections, particularly with the Salvesen family who were to be important clients. Burnett Napier Henderson Orphoot ('Phootie') was born at Peebles in 1880, the son of Thomas Henderson Orphoot, Sheriff Substitute of Lothian and Peebles and his wife Edith Carmichael Smythe Burnett. He was educated at Rugby and the University of Edinburgh and articled to J M Dick Peddie and George Washington Browne, 1900-03, during which period he attended the Edinburgh School of Applied Art. He then spent six months in the office of Robert Rowand Anderson before leaving for France, where he attended the Paris atelier of Gustave Umbdenstock and studied at the Ecole des Beaux-Arts, returning as assistant first to Mewes & Davis in London and finally to Collcutt & Hamp, also of London. From that office he commenced independent practice at 25 Queensferry Street in 1910, and had a London office at 16 Great James Street Bedford Row by 1911. He was admitted LRIBA in the mass intake of 20 July of that year, his proposers being Thomas Edward Collcutt, Stanley Hamp and Charles Henry Gadge.

Orphoot was commissioned in the Royal Engineers in the First World War. While on active service he married in 1915 Marjorie Harriet White, daughter of the Reverend J B White, Instow House, North Devon, where he already had connections, having designed the Clovelly Hotel before the War.

The Queensferry Street office was re-opened after the War, but in 1922 or early 1923 the practice moved to 21 Alva Street and later in the latter year Orphoot formed the partnership of Orphoot & Whiting, and in 1923 that of Orphoot Whiting & Bryce. Of these Frank Edward Whiting was born in 1883 and articled to the London architect Alfred Conder c.1901, thereafter becoming assistant to Detmar Blow and Henry Walter Sarel. By 1914 he was in practice at 30 Bedford Row prior to war service. Whiting was rarely in the Edinburgh office and was based mainly at 21 High Street, Bideford where he held the office of Warden of the Long Bridge. William Theodore Percival Bryce was a pupil of Burnet who had become an assistant in Mewes & Davis's practice and was recruited to run Orphoot's Paris office. For reasons which are not yet clear Bryce withdrew from the partnership in 1932 to recommence practice with his architect wife Helen Mary Bryce, enabling Orphoot to take Lindsay into partnership.

Although Lindsay had joined Orphoot's practice, his relationship with Fairlie remained very close. After Fairlie received the commisson for the National Library in 1934, Lindsay accompanied him on a study tour of Scandinavian libraries. In the course of this trip Lindsay made many influential contacts in architecture, conservation and museums, and was particularly impressed by the work of the St Erik Society in Stockholm. On their return Fairlie recommended Lindsay to the Marquess of Bute as the best person to complete the National Trust's lists of houses and cottages worthy of preservation, lists which had hitherto been compiled by the Marquess himself with the aid of George Scott Moncrieff. It is not clear exactly when he took over, but by September 1936 Lindsay produced a list of 103 towns and villages to be surveyed including the 15 already listed by Bute. By 1938 1,168 buildings had been listed, categorised A, B and C on the model of Amsterdam's non-statutory city list of 1930. Despite the best efforts of John Wilson and his assistant Robert Hogg Matthew, no central funding for the improvement of these buildings was on offer from the Department of Health for Scotland under the 1930 Housing Act but the lists put the activities of the National Trust for Scotland on a truly national footing and led to the foundation of preservation trusts in Inveresk and St Andrews. The latter was founded in 1938 when he first met Ronald Gordon Cant with whom he began a series of small books on the old architecture of the Scottish burghs, published by Oliver & Boyd.

Although Lindsay was a High Church Episcopalian, these activities brought commissions for the restoration of Iona Abbey from the Iona Trustees (on the insistence of J S Richardson as principal inspector of Ancient Monuments) and that of Canongate Kirk and Manse, both in 1938. These were followed by the first of his tower house restorations, Aldie Castle, Kinross-shire in 1939. From that time onwards the Scottish end of the Orphoot practice began to become predominantly conservation based: Lindsay's clientele formed almost a separate entity within the Alva Street office which was otherwise principally concerned with very smart modern houses. The former Caldey monk, artist, historian and writer Peter F Anson became an important member of Lindsay's circle collaborating with him on the design of St Finnan's RC Church, Invergarry (1938) and thereafter making the presentation watercolours of his new-build projects in a style close to that of the Roman Catholic priest-architect John Cyril Hawes. These were very linear with pale washes, but they did reflect the unflashy breadth of treatment and simplicity Lindsay aimed at in his new-build commissions.

From 1936 when Maysie Lindsay found her husband overworked and put an advertisement for an assistant in the Evening News without telling him, Lindsay was most ably supported by George Hay. Born in Edinburgh on 5 July 1911, the son of a skilled metal worker, Hay was educated at the James Clark School and became an apprentice draughtsman with Scott Morton & Company at the age of fourteen in 1925. There he came to the notice of Lorimer & Matthew to whom he transferred in 1928 as an architectural apprentice, taking evening classes at Edinburgh College of Art. At Lorimer & Matthew's Hay spent much of his time on Kings Buildings and at the end of his apprenticeship in 1933 he transferred to the Scottish headquarters of the Office of Works working mainly on the ancient monuments estate under J Wilson Paterson and J S Richardson. His time there had not been altogether a happy one because of Paterson and Richardson's 'silly quarrels' but with Lindsay he quickly developed a close working relationship, his superb draughtsmanship becoming vital to the work of the office. Although their physical appearance and background were so very different - Lindsay was well over six feet tall, his friends mainly landed gentry, Hay was in Ronald Cant's words 'small, neat and purposeful' with strongly held opinions - they had other shared interests; the self-taught Hay had learned Gaelic and several Continental languages the better to understand Scotland's relationship with northern Europe in medieval and Renaissance times, and both had an intense interest in all things traditionally Scottish. It was in Orphoot Whiting & Lindsay's office that Hay completed his studies and was admitted ARIBA in 1937, with a distinction for his thesis on Scottish Architectural Woodwork of the sixteenth and seventeenth centuries. He was very much a Scottish Renaissance man.

The outbreak of the Second World War brought about the dissolution of the partnership with Whiting and the closure of the Alva Street office in 1940. Orphoot continued the practice from his house in Easter Belmont Road until the end of the war, exhibiting earlier projects thereafter to maintain his associateship of the Royal Scottish Academy to which he had been elected in 1934. Lindsay and Hay were called up for military service, Lindsay serving in the Royal Engineers and for a time based in Wales. He ended the war as Major Lindsay, based with the British Army of the Rhine repairing war damaged buildings and constructing temporary hutments to accommodate homeless Germans. Although recommended by Fairlie to the Amenity Committee of the Hydro Electric Board in 1943 as 'the best of the younger architects' and informally appointed chief investigator of historic buildings under the Town & Country Planning (Scotland) Act in March-April 1945, he had difficulty in obtaining his release which was initially scheduled for February 1946. The intervention of Sir Iain Colquhoun and the Department secured his early release on 12 November 1945.

On his return Lindsay re-opened the practice from the basement of Houstoun House, which he had bought from the Shairp family in that year, Orphoot retaining most of his own office records at Easter Belmont Road. Hay returned to the office having served with the Gordon Highlanders before being commissioned in the Royal Engineers. His travels had included North Africa, Sicily, Italy and finally Austria, greatly widening his knowledge of European architecture.

Lindsay was admitted FRIBA on 29 November 1949 and the partnership of Orphoot & Lindsay was finally dissolved in 1952 when Orphoot retired. Hay and another assistant of Orphoot and Lindsay's, Walter Schomberg Scott, were taken into partnership as Ian G Lindsay & Partners, based in the ground floor and basement of 17 Great Stuart Street, which had been Playfair's office, and later Lorimer's, and had been bought by Maysie Lindsay in 1956. A friend of Lindsay's, D Alan Stevenson, last of the harbour and lighthouse engineering family, lived in the floors above. Anson still did Lindsay's presentation perspectives and for a time lived in the basement at Houstoun. The practice expanded rapidly, mainly on church, country house and hydro-electric work, but was not without problems. Lindsay was over-committed as chief investigator of historic buildings and from 1947 onwards was a member of the Ancient Monuments Board and as a member of virtually every body concerned with Scottish heritage; and although most of it was done at weekends he necessarily spent a lot of time visiting and socialising with clients and on researching and writing his books. Hay had more work than he could do and sometimes became stressed; most seriously Scott's clientele was something of a separate entity within the practice and Lindsay and Hay were not always happy with his work, either aesthetically or technically. Like Lindsay, Scott came of a very influential background. Born 14 September 1910 at Monteviot House, Roxburgh, he was the son of James Cospatrick Hepburne Scott, 2nd son of the 7th Lord Polwarth and Lady Isobel Alice Adelaide Kerr, daughter of the 7th Marquess of Lothian. He had studied at Edinburgh College of Art in 1930-35 and had worked in the office of Reginald Fairlie in 1934-35. During these early years he travelled extensively in Italy, Holland, Germany Sweden and France. In 1936 he moved to London to work for T A Darcy Braddell of Deane & Braddell, but returned to Edinburgh to work for Orphoot Whiting & Lindsay in 1937. His home address was then Broomlands House, Kelso. He was admitted ARIBA in that year, his proposers being Fairlie, Braddell and Orphoot. Later he found employment in London with Edward Maufe by whom his work was for a time strongly influenced. In person he was a small and slim man with a military moustache. His marriage on 15 February 1945 to Deborah Castle, a grand daughter of ___ Howard of Castle Howard, extended an already very wide range of social connections.

Although Lindsay was then still serving in Germany, his National Trust (or 'Bute lists') had been adopted by the Department of Health in 1945 as the Town & Country Planning Acts had made it a statutory requirement to compile lists of buildings of special architectural or historic interest. It was quickly realised that the Bute lists were too narrow in scope and in 1947 Sir Robert Russell, who had returned from India as an assistant secretary at the Department, decided to have the necessary resurvey carried out by retired or under-employed architects, following the criteria which had been set out by the Maclagan committee in London. For them Lindsay produced 'the child's guide' 'Notes for the Guidance of Investigators' issued in June 1948 which was considerably in advance of the English instructions in respect of Victorian and Edwardian architecture. The recruitment of part-timers, not all of whom proved to be elderly, greatly widened the Lindsay network, by far the most important of the early investigators being the Aberdeen architect Alexander George Robertson Mackenzie whom he had known before the war. Although Mackenzie was much older, he had a similar outlook on contemporary design and Lindsay was considerably influenced by his thinking. Other early investigators were Alan Reiach in Edinburgh; Robert Alison Crighton Simpson in Duns, a friend from his Cambridge days; Roy Carruthers-Ballantyne in Inverness; John Needham in Dundee; Joseph Weekes in Glasgow and Dumbarton; Antony Curtis Wolffe in the south west; and William Murray Jack in Fife; and after Weekes's death Alfred Lochhead was entrusted with Glasgow and Renfrewshire in 1955, while Catherine Holway Cruft took over Edinburgh from Reiach in 1956. Only Lochhead and Cruft carried out any serious research beyond what was strictly necessary for statutory purposes. Nevertheless the survey was the background to his 'Georgian Edinburgh' (1948) and 'The Scottish Parish Kirk' (1960) while the parallel researches of George Hay resulted in Hay's much more comprehensive 'The Architecture of Scottish Post Reformation Churches', published in 1957. Lindsay and Hay both had a particular interest in the Roman Catholic churches of this vintage, restoring St Mahew's Kilmahew in 1953-55 for the traditionalist historian priest Father David McRoberts who was very architecturally minded. He became an important member of Lindsay's circle and was behind the commission for a large church in Greenock which was sadly never built.

Problems with Schomberg Scott's restoration work for the National Trust for Scotland at The Sandhaven, Culross, and a church with roof problems at Barrow-in-Furness led to the dissolution of the partnership with Schomberg Scott. His place was taken by John Herdman Reid, principally to deal with the new-build side of the business and bring it more in tune with the times. Symptomatic of the change was the laying up of Lindsay's Rolls Royce coupe (its top speed was only 55mph) and its replacement by a Jaguar coupe. But John Reid and George Hay were very different people: and in 1960 Hay regretfully withdrew to return to the Ministry of Public Buildings and Works to be sure of retaining Lindsay's friendship which had begun to seem increasingly at risk. Hay's decision was deeply regretted by Lindsay who more than once observed 'Can't say I miss Scott but I miss Wee Doddie (Hay) dreadfully'. Fortunately by that date the main work on the drawings for Iona and Pluscarden had been done.

In 1962-63 Lindsay undertook a study tour of Australia to advise the Government of Australia on listing and conservation. He was invalided off the return voyage with glandular fever, and shortly afterwards broke a leg struggling with a ram while trying to get it into a boat in Mull. He never recovered properly from either of these mishaps; Hodgkin's disease set in and was not at first recognised for what it was. His last years were also clouded by the loss of the National Trust for Scotland as a client, the work being taken over by the Trust's surveyor under Robert Crozier with Schomberg Scott as consultant, an appointment which particularly upset him. Eventually he became bed-ridden at Houstoun. Although in great discomfort he sat up in bed, received clients and visitors, many of them from the conservation world, and generally did as much as he could. Eventually he had to be moved to Bangour Hospital where he died on 28 August 1966. His funeral took place on the 30th at St John's Church, Princes Street where he had remained a member, and in accordance with his wishes his ashes were scattered to a piper's lament from a boat in Iona Sound. He left £91,579 and was survuved by his wife, three sons and three daughters.

Lindsay's great book on Inveraray, for which Mary Cosh was initially the researcher, was unfinished at his death. It had grown out of his restoration work of the town of Inveraray for the Ministry of Public Building and Works and on Inveraray Castle for the 11th Duke of Argyll. It eventually appeared in their joint names in 1973.

Lindsay was elected ARSA in 1947 and full academician in 1960. He was a big man in every sense of the word and his friends tended to be big men too: a gathering in the office tended to be like a meeting of Sir Walter Scott's six foot club, Lord Macleod of Fuinary being a particularly commanding presence. Lindsay was rarely seen in a suit; he wore blue denim shirts, an immensely heavy vegetable-dyed kilt and a plaid as his everyday business clothes. He always carried a walking stick and he took snuff: this was a great hazard to drawings on linen when he sneezed. For these sneezes he always had enormous red spotted handkerchiefs which, like the shirts, came from a fisherman's shop in Leith. The customs of the old Scots gentry like supping brose standing up, were everyday habits of life; and like Mackenzie's Bourtie, Houstoun was carefully maintained in a state of genteel old-world charm in which the preservation of the original paint surfaces and fittings were paramount considerations. Such concern for original surfaces extended to the waterbound red gravel drive, of which he observed, 'I know my drive looks like a gutted rabbit but it is 'correct'.'