The exterior of St Martin's reflects the Gothic Revival interpretation of the Early English style associated with 13th century churches. Its most notable exterior feature is its considerable height, accentuated by the modest height of the majority of the surrounding buildings; brown brick, offset in some places with red, predominates on the outside, and the layout of the church is standard: an aisled nave with chapels on each side, and a chancel. A saddleback tower was included in the original plans but never built: a small bell-arch with a single bell was built instead, towards the north end. The church is on a north-south (liturgical east-liturgical west) alignment parallel with the Lewes Road.