Yes, in most of Hatfield, with two places where the rules tighten. The town has 96 listed buildings. It has two conservation areas. Nearly a thousand planning applications go in a year. Most of Hatfield is a post-war town that people alter all the time, and the old part is where the paperwork starts to multiply.
Historic England's list holds 96 listed buildings in the Hatfield area: 3 at grade I, 7 at grade II* and 86 at grade II. Listing covers the whole building, inside and out, so changing a listed home needs listed building consent on top of any planning permission, and that includes work that would otherwise need no permission at all. Grade I and II* are the top tiers and draw the closest scrutiny. Most of Hatfield was built after the town was designated a New Town in 1946, and a 1950s semi is rarely anybody's heritage project.
There are 2 conservation areas in the area, Old Hatfield and Sleapshyde. Inside them, the council has more say over changes that affect how a street looks, and some work that would be permitted development elsewhere needs an application. The New Town planners passed over Old Hatfield as the town centre because of its intimate village character, and the conservation area is there to make sure it keeps it.
Hatfield's planning register logged 2,181 applications between April 2024 and June 2026, with 998 in the last twelve months alone. That is a town altering itself steadily, from extensions to change-of-use. Hatfield was once built from scratch by a development corporation; these days the residents redraw it one application at a time.
The ground needs as much thought as the planners. The British Geological Survey rates shrink-swell clay in the Hatfield area at class 3, and about 99% of the area reaches that class. Clay that swells when wet and shrinks when dry can move foundations, especially near trees, so an extension here usually means deeper foundations than a buyer might budget for and a structural engineer's view on the design. It is a cost to plan for, and one that a builder who knows the area will raise before you do.
In practice, a house outside the two conservation areas and off the listed register can often use permitted development rights for modest rear extensions and loft work, subject to the national limits and to any conditions the council attached when the house was built. Inside Old Hatfield or Sleapshyde, or in a listed building, assume an application. The useful check is the council's planning map and the national listing register for the exact address, since the boundary of a conservation area can split a street.
Freedom to extend in Hatfield is middling: most of the post-war town gives owners room to build, while the old town, the listed buildings and the clay underneath all expect to be consulted.