Yes, in most of Billericay you can extend, and plenty of people do. The town has a few dozen listed buildings and three conservation areas. Nearly all of the housing is post-war or newer and carries no special protection. The tighter rule is underground: every postcode is on clay that moves.
Billericay has 67 listed buildings. One is Grade I, four are Grade II*, and the other 62 are Grade II. Listing covers the whole building, inside and out, so almost any change to a listed home needs listed building consent from the council on top of any planning permission, and unauthorised work is a criminal matter. For a town first recorded in 1291, 67 is a light load. Seven centuries of history have left the paperwork on a small number of buildings.
Billericay has three conservation areas: Billericay Town Centre, Great Burstead and Little Burstead. Inside them, the council looks harder at anything that changes how a street looks, such as front extensions, roof alterations, cladding and demolition, and work on trees usually has to be notified first. Two of the three are named after villages, not the town, so moving out to the quieter southern edge can mean moving into more paperwork.
There is one scheduled monument in the area. That is the strictest tier of protection, and it matters mainly if your land is next to it.
Basildon Borough Council recorded 2,274 planning applications in the Billericay area between April 2024 and June 2026, and 1,018 of them came in the last twelve months. That is a lot of residents deciding that building on was easier than moving. The count shows how many people applied. It says nothing about how many were approved.
The ground is the bigger constraint. The British Geological Survey rates shrink-swell clay in its published class 3 across 100% of the area's postcodes. Billericay stands on London clay, which swells when wet and shrinks when dry, and an extension built on it needs foundations designed for that movement. Expect a structural engineer to set the foundation depth, and expect them to dig deeper than a builder's first quote assumed. The planners care about the front of the house. The clay cares about all four corners.
The council-tax valuation list shows only about 6% of Billericay's homes were built before 1930, so most houses here are later stock outside the listed and conservation boundaries. For those homes, many rear extensions, loft conversions and outbuildings can go ahead under permitted development without a full planning application, within national size limits. Confirm whether the house is inside one of the three conservation areas, then ask the council for a lawful development certificate before any work starts. A builder saying the work is permitted development won't count for anything if the council disagrees.
Freedom to extend in Billericay is middling. Most of the town lets you build, a few streets and the two villages are stricter, and the clay sets the cost of the foundations everywhere.