Not without a conversation, and possibly not at all. There are 743 listed buildings inside this small area. There are more scheduled monuments here than most counties manage. The islands sit almost entirely inside a designated landscape. People do still alter their homes here, in reasonable numbers, but they do it with drawings, a heritage argument and patience rather than with a builder booked for the spring.
The listings are the first hurdle. Historic England records 743 listed buildings within the area: 7 at grade one, 30 at grade two star, and 706 at grade two. Listing covers the whole building, inside and out, not just the pretty elevation facing the harbour. Replacing windows, opening up a wall, adding a rooflight, lifting a floor or fixing solar panels to a roof slope all become listed building consent questions, and doing the work first and asking later is a criminal offence rather than a fee. On an island where granite, slate and small sash windows are the vernacular, the answer to "can I have something double glazed and modern" is usually a longer conversation than anyone budgets for.
Conservation areas add a second layer. The area's bounding box takes in 8 conservation areas, one of which is the Isles of Scilly designation itself. Inside a conservation area the planning authority takes a view on demolition, on trees, and on whether what you propose preserves or enhances what is already there, and some of the permitted development rights an ordinary homeowner relies on can be withdrawn. The practical effect is that a change you would make without thinking on a mainland estate becomes an application here.
Then there are 263 scheduled monuments within the same area, which is the strictest protection tier in the system. That matters less for an ordinary house and a great deal for anything involving ground: foundations, drainage runs, a new outbuilding, or a garden project on land near one of them. Scheduled monument consent sits separately from planning permission and is not a formality.
None of this means nothing happens. The planning record shows 170 applications lodged across the area between April 2024 and June 2026, with 80 in the last twelve months of that window. People here extend, convert, reroof and rebuild. The volume is real evidence that the system is navigable, and also evidence of how much paperwork a place of this size generates when nearly every building is protected.
Four listed buildings in the area appear on Historic England's Heritage at Risk register. That is a condition record rather than a warning: a building in poor repair, which may attract grant support for the right scheme and will certainly attract close attention to any proposal. If the house you are buying is one of them, the repair obligations are the purchase, not an extra.
For a buyer, the practical position is that permitted development is the exception here rather than the default assumption. Nearly the whole area sits inside the Isles of Scilly Area of Outstanding Natural Beauty, which is another designation an officer reads before they open your drawings. Before offering on anything, establish whether the property is listed and at what grade, whether it sits within a conservation area, and what has already been approved on it, because that history tells you more about what you will be allowed than any general rule will.
Freedom to alter a home is the most constrained thing about these islands. Buy the house you want to live in rather than the house you plan to improve, because the version of it that exists today is close to the version you will be keeping.