Greatham Church is a tiny, primitive, unrestored building dating from the "Saxo-Norman overlap" period when Anglo-Saxon architecture was giving way to the Norman style. The writer Arthur Mee was once told not to mistake the church for a haystack: set in the middle of fields and on its own apart from the manor house, it has an extremely basic and modest feel—amplified by its lack of electricity and piped water. The walls, of unequal length and thickness, are principally of ironstone with some chalk, flint, Roman-era masonry and rubble, local (Pulborough) sandstone and Horsham Stone. Ian Nairn described them as having "a wonderful texture". At one corner, the Pulborough sandstone quoins have a set of roughly carved crosses. These are believed to be consecration crosses, which were blessed when the church was completed and consecrated. The building has a single room with no division between the nave and chancel. There is also a slate-hung bell-turret with a small spire. The windows, inserted in the 13th century, are Early English-style lancets. The roof is an old, but not original, king post structure.