You can extend most houses in Radlett, but the ground and the conservation areas will both have a say. Very few homes are listed. Two conservation areas cover parts of the town. The clay under every postcode means deeper foundations. The neighbours are applying for planning permission all the time, so the system is plainly working for someone.
Radlett has 17 listed buildings: none at grade I, one at grade II* and 16 at grade II. Listing protects the whole building, inside and out, so changes to a listed home need listed building consent from the council as well as any planning permission, and the process is slow and detailed. Seventeen is a short list for a town this size. Radlett demolished Newberries mansion in the 1950s and Aldenham Lodge in 1964, and that did wonders for the listing count.
The town has two conservation areas, Radlett North and Radlett South. Inside them, the council looks harder at anything that changes how a house looks from the street, such as extensions to the side or front, dormer windows, new boundary walls or felling trees. Some work that would otherwise go ahead without an application may need one. Look up the house on Hertsmere's conservation-area map and ask the sellers for its planning history, because buyers need to know which of the two Radletts they're buying into.
Plenty of people still apply. Hertsmere's planning records show 758 applications for Radlett between April 2024 and June 2026, and 313 in the last twelve months. That counts applications made, not how many were granted, but it shows people here change their houses all the time and expect to get somewhere with it. A town that can't build outwards builds onto what it already has.
The bigger issue for an extension is underground. The British Geological Survey puts Radlett's shrink-swell hazard at class 3, and 100% of the area reaches that class. Shrink-swell is clay that swells when wet and shrinks when dry, which moves foundations that are too shallow. For an extension, that means a structural engineer's view early on and foundations dug deeper than a builder might quote off the cuff, especially near trees, which pull water from the clay. The Romans came for this clay and ran potteries on Loom Lane, and their pits are still visible at the southern end of the village. The clay hasn't gone anywhere, and it now has opinions about your rear extension.
Outside the conservation areas and away from the listed buildings, many ordinary extensions to an ordinary house can go ahead as permitted development, without a full application. Inside the conservation areas, or on a listed building, expect to apply. Either way, a lawful development certificate from the council turns "probably fine" into something written down.
Your freedom to extend in Radlett is middling. The rules only bite hard on a small part of town, and the clay underneath the whole of it is what will drive the cost.