Yes, for most homes in Colney Heath, though a few of them come with paperwork and almost all of them sit on clay. Listed buildings are few and almost all grade II. One conservation area covers one hamlet. The neighbours file planning applications at a steady clip. The ground is the bigger question for most extensions.
The area has 45 listed buildings. None is grade I, 1 is grade II* and 44 are grade II. Listing covers the whole building, inside and out, so changes that affect its character, from an extension to replacement windows, need listed building consent on top of any planning permission. For a parish full of post-war semis, 45 is a small number, and most buyers will never own one.
There is 1 conservation area, at Sleapshyde. Inside it, the council takes an interest in changes that would pass without comment elsewhere, and demolition, some alterations to the front of a house and work to trees can need consent. A house in Sleapshyde comes with more rules than one in the rest of the parish.
The planning register for the area holds 1,171 applications made between April 2024 and June 2026, with 543 of them in the last twelve months. That is plenty of evidence that people here do extend, convert and alter their homes. Somebody near any given house has almost certainly been through the process recently.
The British Geological Survey puts the ground across the area at shrink-swell class 3, and about 92% of postcodes reach that class or higher. Clay that swells when wet and shrinks when dry can move shallow foundations, so an extension here may need deeper footings than the builder's first quote assumed, and a structural engineer should look at the soil and any nearby trees before the design is fixed. The ground under Colney Heath moves with the seasons, and any extension has to be built to allow for that.
For an ordinary house that is neither listed nor in Sleapshyde, many single-storey rear extensions and loft conversions can go ahead under permitted development rights, within size limits, without a full planning application. Listed buildings and conservation-area homes lose some of that freedom and need consent. Ask the council for a certificate of lawful development for the specific project at the specific house, because a builder saying it is permitted development is no protection if the council later disagrees.
Colney Heath gives middling freedom to extend: the planning rules are light outside Sleapshyde and the listed buildings, but the clay asks for more care and money than most buyers plan for.