Little Ben McCreath had run on ahead of his mum Louise McCreath, who was 29 at the time of the accident. She had been at a meeting at her employers, law firm Ledingham Chalmers, in the Capital's Princes Exchange building in Earl Grey Street.
Mark McPhilips, 47, an architect who had been involved in designing the building, said the gap had been increased from 100mm to 150mm.
Mr McPhilips was asked if 150mm was the limit to what he would find acceptable.
He replied that the gap would not have been made as large as, for example, 500mm, but he had changed it to solve a problem.
Edinburgh Sheriff Court yesterday heard Mr McPhilips could not recall what the problem was.
The fatal accident inquiry has heard evidence that in buildings where children are anticipated to be present, a rule applies to prevent gaps being big enough for a youngster to fall through.
But the court heard this rule did not apply to Princes Exchange as it was an office building.