The Washington Naval Treaty of 1921 did not mention minor vessels, and so this class was put in hand to replace the older sloops, and were little more that modern versions of the "Old Flower" class. The London treaty however, declared vessels under 2000 tonnes, with smaller than 6" guns and less thatn 20 knot speed were not restricted, so this formed the limiting factor of this class, and all subsequent sloops.
By adopting more modern machinery ( the turbine ) and construction, a saving of several hundred tons was made, and with oil fuel, an equal speed but much greater radius was gotten.
Armament was to be a 4" HA gun forward, and a similar LA gun aft, and because the "Old Flower" were excellent minesweepers, minimum draft was worked in so they could revert to this role. They were also capable ocean escorts, the role of the sloop at that time being more along the lines of colonial duties - 'send a gunboat' policy.
The Shoreham and Falmouth groups were 15 ft. longer than the original six units.
War modifications included standardising on 4" AA guns, adding second quad machine gun aft, two 20mm cannon in the wings, SW type 271 and AW type 291 RDF, and later both machine guns were replaced in some by 20mm cannon. Sonars were also updated, an a hedgehog DC launcher was added forward.