Usually, though rarely the version you sketched on the back of the brochure. Around forty buildings in the parish carry a listing. The village core sits inside a conservation area. Neighbours are putting applications in at a steady rate, so this is a place where alteration clearly happens rather than one where it is forbidden. The question is not really whether you can build. It is how much of the design the planning department ends up owning.
Historic England's register records 39 listed buildings here. One is grade two star, 38 are grade two, and none are grade one. Listing is not a plaque, it is a legal control: alterations that affect the building's character need listed building consent, and that applies inside as well as out, to windows, staircases, chimneys and fabric you might reasonably assume were yours. Owning a listed house in Calstock means the cheap answer and the permitted answer are frequently different answers, and the permitted one wins.
The village also has its own conservation area, one designated area covering the historic core. Designation brings an additional layer of control over changes that affect the appearance of the place, particularly anything visible from the street, and the precise scope is set by Cornwall Council rather than by anything national. Worth adding that every postcode in the parish sits inside the Tamar Valley Area of Outstanding Natural Beauty, which is part of the backdrop any application here is judged against.
None of which stops people. Cornwall Council's planning register records 156 applications across the area between April 2024 and June 2026, with 75 of those in the last twelve months. That is a village where somebody is always dealing with an architect, and it is the most useful evidence available that extending here is a normal thing to do rather than a heroic one.
The ground carries its own paperwork. Every postcode in Calstock has abandoned mine workings within 500 metres, with the nearest averaging 184 metres, and six postcodes sit directly over a recorded working. This is what a copper and arsenic valley leaves behind. In practice it means a mining search as standard, possibly a specialist report on top, and the real-world consequence is usually a slower transaction and a lender or insurer asking questions rather than a structural problem. It is a due diligence item, not a verdict on the house.
The practical position is this. If your house is unlisted and outside the conservation area boundary, a modest rear extension may fall within permitted development and you should confirm that with the council rather than with an internet forum. If it is listed, inside the conservation area, or both, assume consent is needed and budget the time before you budget the kitchen. Anyone buying here with a firm plan should test it with the planners before exchange, not after the builder has quoted.
Freedom to alter a house in Calstock lands somewhere in the middle. This is not a locked-down heritage museum, and plenty of people are getting work through, but between the listings, the conservation area and a hillside full of old workings, the paperwork will always arrive before the scaffolding.