You can, but expect the paperwork to take longer than the building. There are hundreds of listed buildings in and around the village. Conservation areas cover the surrounding villages. The clay under almost all of it shrinks and swells. Extending here is possible. It just isn't simple.
Historic England lists 291 buildings in and around the Hadlow area: 8 at Grade I, 16 at Grade II* and 267 at Grade II. If your house is one of them, listed building consent is needed for works that affect its character, inside as well as out, and that covers more than most owners expect: windows, internal walls, roof materials, even how an old wall is repointed. Doing the work first and asking afterwards isn't a shortcut. It's a criminal offence, and the council can make you put it back.
There are 18 conservation areas in and around the area, including Hadlow itself, Fairlawne, Ightham, Ivy Hatch and Claygate Cross at Plaxtol. Inside one, the council takes a closer interest in anything visible from the street: materials, demolition, some trees and the overall look of an extension. The buyer who pictured a glass box on the back of a Kent cottage will be told politely how that is going to go.
Four scheduled monuments sit in the wider area. That is the strictest tier of protection, and it matters mainly if your land is next to one.
The neighbours are not deterred. Between April 2024 and June 2026 the area's planning register logged 3,001 applications, 1,337 of them in the last twelve months. People here do extend, convert and alter their homes. They just do it with drawings, forms and patience.
The ground adds its own condition. The British Geological Survey's shrink-swell hazard mapping puts the area at class 3, and over 99% of the area reaches that class. Clay soils like these swell when wet and shrink in dry spells, which affects how deep an extension's foundations need to go and how close it can sit to trees. It is a matter for the structural engineer and the building control officer, not a reason to panic. The extension that saves money on foundations is the one that ends up paying for them twice.
In practice, a house that is neither listed nor in a conservation area may be able to use permitted development rights for modest changes, depending on the property and its planning history. Once a house is listed or in a conservation area, assume consent is needed and check with Tonbridge and Malling Borough Council before booking a builder. Your solicitor should also confirm whether earlier works on the property had the consents they needed, because the next owner inherits the problem along with the extension.
Your freedom to change a home here is limited. Hadlow is a place to buy a house you already like, not one you plan to remodel.