For most houses in Portishead, yes, extending is well within reach. The town is a modern one, with the bulk of its stock built since 2003 and only a small fraction predating 1930, which means the average home carries none of the heritage baggage that ties owners in knots elsewhere. Only around 40 buildings across the town are listed. Four conservation areas cover specific pockets rather than the whole place. Over the past year alone the council handled more than three hundred planning applications locally, so this is a town where people extend, convert, and alter their homes all the time, and mostly succeed.
The listed buildings are the exception, and a narrow one. Of the roughly 40 listed structures, just one holds the top grade, three sit in the next tier, and the remaining three dozen are the standard grade. If your home is one of them, listing controls almost everything you might want to do, inside and out, and consent is required for changes that an ordinary owner would never think twice about. That is a real constraint, but it applies to a tiny slice of the town, and the odds are heavily against your house being on the list.
The four conservation areas are the other thing to check. They cover Portishead Court Farm Environs, Portishead West Hill and Welly Bottom, Portishead East Wood, and Portishead Woodhill Road, and within their boundaries the council takes a closer interest in how buildings look and how alterations affect the character of the street. That can mean extra consent for work that would be automatic elsewhere, and some of the rights homeowners normally take for granted may be tightened. It is scrutiny rather than a ban, but it adds time and paperwork to anything you plan.
In practical terms, the split is simple. If your house is unlisted and outside those four conservation areas, which describes the great majority of Portishead, a lot of extending falls under permitted development and never needs a full application at all. If it sits inside a conservation area or wears a listing, assume you will be asking permission and building the timeline around the answer. Either way, checking the status of the specific address before you draw up plans is the step that saves the grief, because finding out after you have paid an architect is the expensive way to learn it.
The blunt verdict is that Portishead gives you middling freedom to alter a home: genuinely easy for the modern, unlisted majority, and properly constrained for the heritage few. Check which side of that line your house falls on, and most buyers will be pleasantly surprised.