Extending a house in Filton is about as unencumbered as English planning gets. There are only thirteen listed buildings across the whole area, every one of them at the lowest grade, and there are no conservation areas and no scheduled monuments to trip over. The vast majority of homes here carry no special heritage protection at all. If you have bought an ordinary interwar semi and you want a rear extension or a loft conversion, the suburb is not going to stand in your way on heritage grounds. This is a place built for living in, not for preserving under glass.
The listing picture is thin and gentle. Thirteen listed buildings is a small number for an area this size, and the fact that all of them sit at grade two, the least restrictive tier, matters. Grade two listing protects a building and requires consent for changes that affect its character, but it applies only to those specific properties, and the odds that your house is one of the thirteen are slim. The heavier protections, the top grades that turn every alteration into a negotiation, do not exist here. Owners of the listed handful will need to tread carefully and get proper consent before touching anything of significance. Everyone else is dealing with the standard rules, not a heritage overlay.
In practice that means the ordinary path is open to you. A home without listing and outside any conservation area falls under the normal planning framework, where a good deal of modest work can proceed under permitted development and larger schemes go through a standard application rather than a heritage one. The conservation-controlled corners, where a change of window becomes a committee matter, sit in neighbouring places like Westbury on Trym and Henbury rather than in Filton itself. Here the constraint is the usual one of building regulations, neighbours and the size of the plot, not a conservation officer guarding a streetscape. As ever, confirm the status of the specific address and check what permitted development still allows before you draw up plans, because the freedom is broad but not infinite.
Filton scores well on the freedom to alter a home, and rightly so. Unless you have landed one of the thirteen listed buildings, the heritage system leaves you alone, and extending here is a question of budget and building regulations rather than a fight over preservation.