Allason was born in 1790 and trained as an architect under William Atkinson. He made the obligatory tour of Roman architectural remains and in 1817 published "Picturesque Views of the Antiquities of Pola in Istria". As an architect, he built several country houses and office buildings. He became the Ladbrokes’ estate surveyor, a role which he also performed for The Stock Exchange Estate and the Pollen and Pitt Estates in Kensington. He was also appointed to various public surveying offices such as the Commission of Sewers.
It was during the lifetime of James Weller Ladbroke that development on the estate began in earnest. Ladbroke had little personal interest in it and left matters in Allason’s hands, while he concerned himself with country pursuits in West Sussex.
Thomas Allason provided the original lay-out plan for the estate in 1823. He proposed a circular road with large houses along its inner side (probably influenced by John Nash’s Regent’s Park). It was to be bisected by a long road, running north to south. But before development could start in earnest, there was a financial slump in 1827 and building more or less ceased.
One feature of Allason’s plans which was carried forward in the actual development on the estate was a novel idea for gardens. In most Georgian developments, houses were built round gardens. The houses faced onto the gardens. Allason turned this round. The houses were to back onto the gardens, which would be private communal gardens. Ultimately 15 such gardens were built on the estate.
In the 1840s Allason took leases of some plots on the estate himself and built houses for profit. He died in 1852, working to the end.