Yes, in most of Letchworth you can extend, as long as the house is outside the conservation areas and isn't listed. The town has around a hundred listed buildings. Four conservation areas cover the original garden city and its old villages. The neighbours put in planning applications all the time. Where your house is decides how many forms you fill in.
Historic England lists 109 buildings in and around Letchworth, 11 of them grade II* and 98 grade II, with none at grade I. Listing covers the inside of a building as well as the outside, so almost any change to a listed home needs listed building consent from North Hertfordshire Council, as well as planning permission where that applies. With no grade I buildings here, nobody is guarding a cathedral. They are guarding a hundred-odd early twentieth-century houses and public buildings from the garden city experiment, which explains why the scrutiny lands so heavily on ordinary homes.
Four conservation areas cover the area: Letchworth itself, Norton, Croft Lane in Norton, and Willian. Inside them, alterations visible from the street get more attention from the council, and demolition and some works that would elsewhere go ahead without an application need consent. Willian has kept a character separate from the town next to it, and its conservation area is there to protect that. North Hertfordshire Council's conservation area maps are the first thing to check for any address, because the boundaries run through residential streets and two houses facing each other can come under different rules.
There are 2 scheduled monuments in the area, the strictest tier of heritage protection. They mainly matter for land next to them, not for an ordinary back garden. Bronze Age builders put a hill fort on Wilbury Hill without asking anyone, and anyone digging near ancient remains here today needs permission first.
The planning register shows how active the town is. There were 1,395 applications between April 2024 and June 2026, and 672 in the last twelve months of that period. In 1905 and again in 1907 the town held exhibitions inviting builders to try out new ideas for cheap houses. The experimenting carries on today, but now it starts with an application.
One listed building in the area is on Historic England's Heritage at Risk register. That means a building in poor condition, whose repair may get grant support and will be watched closely. It doesn't affect anyone buying an ordinary house nearby.
Outside the conservation areas and away from listed buildings, many rear extensions and loft conversions in Letchworth can go ahead under permitted development rights, the national allowance for modest work without a full application. The terraces and post-war semis that make up most of the town mostly have those rights. Inside a conservation area the allowance is narrower, and on a listed house almost nothing counts as minor. The same plan for the back of the house can need no application on one street and a full one on the next.
Your freedom to extend in Letchworth is about average. Outside the garden city's protected core it is ordinary suburbia, and inside it you'll be making your case to the council.