Usually yes, and the exceptions are concentrated rather than scattered. Most of the town is ordinary twentieth-century housing with ordinary permitted development rights. The historic core is a different proposition and has the paperwork to prove it. Hundreds of applications have gone in locally over the past couple of years, so nobody here is under the impression that building work is impossible. The question is not whether you can extend. It is which half of the town you bought in.
There are 90 listed buildings in the area: one at grade one, eight at grade two star, and 81 at grade two. Listing covers the whole building inside and out, not the pretty elevation the agent photographed, which means internal alterations, window replacements and anything structural need listed building consent on top of any planning permission. Doing the work first and asking afterwards is a criminal offence rather than a telling-off, which is the detail most people discover on a forum at two in the morning.
One conservation area covers St Columb Major itself, and it changes the rules rather than removing them. Inside the boundary, permitted development rights are typically trimmed, demolition needs consent, and trees get protection they would not have two streets away. The practical effect is that the things which read as cosmetic, the render, the front windows, the roof covering, are the things that become a formal application. Cornwall Council publishes the boundary, and looking at it takes about four minutes, which is roughly four minutes more than most buyers spend on it.
Three scheduled monuments sit within the area, carrying the strictest protection tier in the system. They matter almost entirely to landowners near them rather than to anyone extending a kitchen in town, but if your plot adjoins one, the consent regime is national rather than local and the council cannot simply wave it through.
Meanwhile, people are getting on with it. Planning records show 806 applications across the area between April 2024 and June 2026, with 369 of those in the last twelve months. That is a town where extensions, conversions and alterations are a normal part of the week rather than a rare event, and it is worth more than any general assurance about planning policy.
Two of the listed buildings locally appear on Historic England's Heritage at Risk register, meaning their condition has deteriorated enough to warrant monitoring. For anyone buying an ordinary house nearby, that is background rather than a problem. For anyone buying one of them, it means both grant funding may be available and every decision will be watched closely, which is a trade most people should think about carefully before falling in love with the staircase.
In practical terms, the split is clean. Modern estate houses come with the standard permitted development rights and the standard rear-extension routine. Older property in the centre, listed or within the conservation area, needs consent for work that elsewhere needs nothing at all, and the professional fees start before the builder does.
Freedom to alter a home here is middling: unremarkable across most of the town, genuinely constrained in the old centre. Check which one you are buying before you start drawing anything.