Yes, and the permission you need depends entirely on which side of a line your house sits. There are 112 listed buildings in the area and three conservation areas. Much of the parish falls inside a World Heritage Site built on the remains of the mining industry. People here extend, convert and reroof constantly. They just do it with a planning officer copied in.
Start with the listings. Of the 112 listed buildings, 110 hold grade two status and two hold grade two star. None are grade one. Listing applies to the whole building inside and out, which means the things owners assume are cosmetic, the windows, the internal doors, a flue, a partition wall, become matters requiring listed building consent. Being listed does not mean nothing can change. It means every change is a proposal rather than a decision, and the answer takes months rather than an afternoon.
Three conservation areas cover St Agnes itself, Mithian and Mingoose. Inside one, the ordinary permitted development freedoms are narrowed: the council can require consent for work that would pass without comment a mile up the road, and trees carry protection by default. What is allowed varies by area and by what the designation was drawn to protect, so the only reliable answer is the one Cornwall Council gives for your specific address.
There are also 11 scheduled monuments in the area, which is the strictest protection in the system and effectively non-negotiable. For most buyers this is irrelevant unless the boundary of one runs near the plot, in which case any groundworks become a specialist conversation before they become a quote.
None of this has stopped the neighbours. Planning records show 309 applications across the area between April 2024 and June 2026, with 133 of those in the last twelve months. For a parish this size, that is a steady stream of people altering their homes rather than a village frozen behind a designation. Whatever the constraints do, they do not stop the work being attempted.
Then there is what the ground is made of. Every postcode in the area has abandoned mine workings recorded within 500 metres, and 157 postcodes sit directly above a recorded working. That is what happens when around a hundred mines once operated in one parish. In practice it means a mining search is standard rather than optional, a specialist report may be asked for, and lenders and insurers will have their own views before you get anywhere near a foundation design. It is a process fact, not a verdict on any particular house, and thousands of Cornish homes carry it without incident.
So the honest version: an unlisted house outside the conservation areas, on a plot with room to the side, behaves like any other house and permitted development will cover a good deal of it. A listed cottage on a terraced hill, or anything within a conservation boundary, is a project where the design fee and the consent timeline come before the builder's quote. Both types are on the market here, and they are not remotely the same purchase.
Freedom to alter a home is one of the weaker aspects of buying in St Agnes, because the things that make the village worth photographing are the same things that generate the paperwork. Find out which designations touch your address before you fall in love with a set of plans.