The short answer is yes, and the planning department is not the part that will fight you. Sandown carries eleven listed buildings in total, all of them at Grade Two, which is the least restrictive grade. There are no conservation areas at all in the town. There is one scheduled monument, a Palmerston-era fort on the western cliffs. The rest of the housing stock is open for the kind of work most home extensions involve.
All eleven of Sandown's listed buildings are at Grade Two. None are at Grade One. None are at the intermediate Grade Two Star. That matters because Grade One and Grade Two Star buildings are the ones that attract the heaviest correspondence from heritage officers and the strictest restrictions on internal alteration. Grade Two listing protects the exterior and any historically significant features from change without listed-building consent. The owner of a Grade Two house can still extend and modernise. The application is doing extra paperwork rather than getting an automatic no. The named list includes four churches, three manor houses and a Roman villa. The town's residential streets do not carry many of them. If you are buying a Victorian terrace or a post-war bungalow in Sandown, the odds your house is on the list are small, but the check still has to be done. A listing search is a five-minute exercise, and it belongs before the offer rather than after.
What this means in practice is that the typical Sandown homeowner is operating in the same permitted-development territory as the typical homeowner anywhere else in England. Single-storey rear extensions within the standard limits and loft conversions that do not break the principal roof slope can usually be built without a full planning application. Anything that exceeds those limits or sits within a few metres of a boundary needs the consent route. The absence of a conservation overlay means the council is not adding extra restrictions on top of national policy. There is no evidence in the public planning information that the town carries any local restrictions beyond the standard. A house that is not listed and not in a conservation area is, by national framework, the most permissive starting point a homeowner gets.
On the freedom to extend or alter a house, Sandown sits above average. The historic protection is concentrated on a handful of named buildings rather than spread across the town. Most homeowners can get on with the kind of project they had in mind without losing months to a heritage objection. The constraint that will slow your weekend is the neighbour, not the planner. The constraint that will slow your budget is the builder.