The N class incorporated the principles of power and reliability established by George Churchward, using a Belpaire firebox that sloped downwards towards the cab instead of a round-topped version, a regulator located in the smokebox, long-travel valves for free running up to , a sharply tapered and domeless boiler, and a right-hand driving position. These features are attributed to Holcroft, who worked on the GWR 4300 class before joining the SECR. The boiler was intended to become a standard component for use on future SECR locomotive designs, thereby reducing building times and improving organisation at the works. The size was constrained by the heavier axle-loading of Maunsell's proposed 2-6-4 tank locomotive variant of the N class, the K class, and was consequently smaller than was otherwise possible on the 2-6-0 chassis. The need to reduce overall weight also meant that the latter would feature lightly braced frames.