Agas's regular work consisted of drawing up local estate maps and surveys for a variety of clients. He was one of the first estate surveyors to move beyond the traditional practice of compiling purely written descriptions of landed property, and to consider supplementing these with measured maps. The earliest map that can be attributed to him is one of lands at West Lexham, Norfolk, dated 1575. He subsequently undertook commissions in Bedfordshire, Berkshire, Cambridgeshire, Essex, Gloucestershire, Norfolk, Oxfordshire, Suffolk and Surrey. An estate map he drew of Toddington, Bedfordshire, dated 1581, includes what Paul Harvey has described as "the best picture we have of a small Elizabethan country market-town". He appears to have been patronised by William Cecil, 1st Baron Burghley. Another client was Corpus Christi College, Oxford, although the College commissioned only written surveys rather than maps.