Yes, you can extend most houses in Gidea Park, though the streets that gave the suburb its name are where it gets difficult. Most of the housing is interwar or later, and most of it can be changed. The Edwardian exhibition streets are protected. The whole area is built on ground that moves with the weather. Plenty of neighbours are already applying.
The area has 25 listed buildings: none at grade I, 2 at grade II* and 23 at grade II. Listing covers the inside of a building as well as the outside, so a new window, a knocked-through wall or a change to the roof needs listed building consent as well as any planning permission. Several of the listed buildings are exhibition houses, built in 1910 and 1911 to show off the best of Edwardian house design, and the council still treats them as exhibits.
Three conservation areas reach into the area: Gidea Park, Romford, and Langtons, Hornchurch. The Gidea Park one covers the exhibition streets, including Heath Drive, Meadway, Reed Pond Walk and Parkway. Inside a conservation area, the council looks harder at anything visible from the street, and some work that would be routine elsewhere needs consent. Ask Havering Council whether the house you are looking at is inside the boundary and whether any extra restrictions apply. On Meadway, that answer probably matters more than the floor plan.
There is one scheduled monument, the strictest tier of protection. It only matters if a house's land runs up to it, and most gardens here don't.
In the 27 months from April 2024 to June 2026, residents submitted 499 planning applications, 221 of them in the last twelve months. The garden suburb itself took about a year to build, and the people living in it have kept applying to change it ever since.
The ground needs as much attention as the planners. Every part of the area is shrink-swell class 3 on the British Geological Survey's hazard mapping, meaning the clay underneath swells when wet and shrinks when dry. That doesn't stop anyone extending. It does mean the foundations for a new extension will probably need to go deeper than a builder's first quote assumes, and a structural engineer should look at the plans before the digger arrives. Big trees near the house can make the effect worse, so the engineer should take those into account too.
For an ordinary interwar semi outside the conservation areas, permitted development rights mean a modest rear extension or loft conversion can often go ahead without a full planning application. Those rights are narrower inside a conservation area, they do not cover listed building consent, and flats, which make up about 37% of homes, have far fewer of them. A house on the exhibition streets will usually need a conversation with the council first.
How free you are to extend here is middling and depends on the street. A semi a few roads away from the exhibition estate can usually go ahead with little fuss, while a house on one of the protected streets will need the council's agreement for most changes.