The 3.5 metre figure your neighbour's planner will use
Ipswich Borough Council judges extensions against its Space and Design Guidelines, adopted in November 2015. It replaced the old 1999 guide, so ignore advice built on that.
The number to remember is 3.5 metres. Ipswich's guidance says a single-storey rear extension should not run more than 3.5 metres past the neighbouring house along, or close to, the shared boundary. Further from the fence, deeper may be fine. For two storeys at the back near a boundary, expect to be held to 3 metres or less.
Light gets tested too. Planners draw 45 degree lines, in plan and in elevation, from the middle of the neighbour's nearest window. If that window centre falls inside both lines, the guidance says a lot of light is likely to be lost. On a California terrace, where back additions already sit close together, that test often decides how far the new kitchen can come out.
Two more local points catch people out. Planners will generally resist a big flat roof added to a house with a pitched roof, though a modest parapet design is sometimes accepted. And an extension should usually take up no more than half of the garden space round the house.
Which extension suits an Ipswich house
Single-storey rear. The first project on most Ipswich semis. Outside the conservation areas you can usually go 3 metres past the original back wall (4 metres if detached) with no application. Prior approval stretches that to 6 or 8 metres, but the council's 3.5 metre boundary figure is what it will weigh if neighbours object.
Kitchen. On an older terrace off Woodbridge Road the old kitchen is usually a narrow back addition. Spend the design effort on rooflights and where the doors open, because the 45 degree test may cap the depth anyway.
Side return. Made for the alley beside a back addition in California or round St Helen's. Inside a conservation area, though, infilling past the side wall needs planning permission even at one storey.
Wrap-around. Part of it sits beyond a side wall, so it needs an application in any of the 15 conservation areas. Elsewhere the side portion must stay single storey and no wider than half the house.
Open plan. Opening the back of a 1930s semi in The Dales or Castle Hill means a steel beam and new pad foundations. On London Clay the engineer will want to know about nearby trees before sizing them.
Two storey. With semis making up 38.5% of Ipswich households, this is mostly a side addition to one half of a pair. The council resists unbalancing a semi and asks for the upper floor to be set back from the front, by up to 4 metres in some cases, so the gap between houses doesn't close up.
Downstairs bedroom. The hipped 1930s bungalows of The Dales lend themselves to this. Check flood risk first if you're near the river, since a ground-floor bedroom is the room you least want water in.
Home office. Attached is easier to heat and wire. Keep any side window from looking straight into next door's garden, which the planners will pick up on.
Planning. Before paying for drawings, find out if the house sits in a conservation area, an Article 4 street or Flood Zone 2 or 3. The answers shape everything after.
Cost. No figures here. What moves an Ipswich quote is the depth of foundation the clay needs, steel, and whether you need a flood assessment. Give three builders the same drawings and compare like with like.
Clay, chalk and old marsh under the foundations
The British Geological Survey's notes on the Ipswich district are worth five minutes. London Clay changes volume as it wets and dries, and the survey says trees play the dominant role in that. A big tree near your planned back wall is a foundation question. Some clays also carry pyrite, which can produce sulphate that attacks ordinary concrete, so your engineer may specify a resistant mix.
Chalk turned up in many of the boreholes sunk for site investigations across town, and the survey warns it can dissolve and leave voids. Down by the lower Gipping much of the ground is made ground, laid over old marsh to extend the port, with soft silt and peat in the valley. Building control will look at the dig before the concrete goes in.
Flood checks near the Gipping and the Orwell
The tidal barrier on the Orwell opened in 2019, and the council now calls tidal flooding a residual risk. That doesn't take your house out of the flood map. Its flood guidance says any development in Flood Zone 2 or 3, minor work included, needs a flood risk assessment with the application, as does building in one of the critical drainage areas named in the town's surface water plan.
Surface water is the bigger worry for most families. Ipswich's 2020 flood study names Norwich Road, Bixley Road, Holywells Road, Maidenhall and Cobham Road among its worst spots, and says some older soakaways have proved inadequate. More roof means more rainwater, so ask where it will go before the drawings are fixed.
Two quick notes on the wider rules. There's no Green Belt round Ipswich, so there is no percentage cap on how much you add. In the Anglesea Road, Norwich Road and Marlborough Road Article 4 areas, any enlargement facing a road, waterway or open space needs permission. On a corner plot that can include the back.
Where these facts come from
- Ipswich Borough Council: Space and Design Guidelines SPD (2015)
- Ipswich Borough Council: Space and Design Guidelines SPD adoption
- Ipswich Borough Council: Development and Flood Risk SPD, practical guidance
- Ipswich Borough Council: Development and Flood Risk SPD
- Ipswich Strategic Flood Risk Assessment (2020)
- British Geological Survey: geology of the Ipswich district
- Ipswich Borough Council: Anglesea Road/Norwich Road Article 4 direction
- Ipswich Borough Council: Plan 3 Conservation Areas
- Ipswich Housing Report 2024 (Census 2021 housing mix)
- GOV.UK: Local authority Green Belt statistics 2024-25
- GOV.UK: Permitted development rights for householders, technical guidance
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Questions people in Ipswich ask
Our neighbour in California has a 4 metre extension. Can we match it?
Possibly, but don't assume. The council's guidance looks for single-storey rear additions to stay within 3.5 metres of the adjacent house along the boundary, and it tests light to their windows. Their extension changes the picture for yours, so ask a designer to check both.
We're on Bixley Road. Does surface water flooding affect an extension?
It can. Bixley Road is on the flood study's list of the worst surface water spots. If your plot sits in a critical drainage area a flood risk assessment goes with any application, and your builder should plan where the extra roof water drains.
Can we put a flat-roofed extension on our 1930s semi in Westbourne?
A small one, often yes. Planners generally resist large flat roofs on houses with pitched roofs, and is more open to a modest design with a parapet. A low-pitched tiled roof matching the house is the safer route.
There's a willow at the end of our garden. Will it affect the foundations?
Quite possibly. The BGS says trees are the main driver of movement on London Clay, and willows are thirsty. A trial hole will show what's under you, and an engineer will set the foundation depth from the tree's species, height and distance.