House Extensions Letchworth Garden City
Single storey rear and double storey extensions in Letchworth Garden City, planning, structure and build under one contract.
- Rated 4.9 from 31 reviews on MyBuilder
- Guide from £40,000 (London-derived)
- Free site survey, itemised quote
An extension in Letchworth Garden City can need two separate approvals from two separate bodies. We survey, appoint a structural engineer, work alongside your architect or designer on the planning side, coordinate the building control application, and build the shell, the roof and the internal work. We never produce structural calculations. This page sets out both approval routes, the design tests your drawings are judged against, and the ground conditions.
For Letchworth Garden City, on London-derived ranges a single-storey rear extension typically starts from £40,000, or £2,000 to £2,800 per m²; Hertfordshire work generally runs below that. GS Renovation Ltd, a building contractor based in Potters Bar, does the work in Letchworth Garden City: the structural engineer we appoint designs the structure, we build it, and building control signs it off.
Price guide
| Work | Indicative range |
|---|---|
| Single-storey rear extension | from £40,000 |
| Rear extension, per m² | £2,000 to £2,800 |
| Side return, per m² | £2,400 to £3,200 |
| Wraparound, per m² | £2,400 to £3,200 |
| Double storey, per m² | £1,800 to £2,600 |
| Basement extension | see the basement page |
Indicative London ranges (Architecture for London, Resi, MyBuilder and Checkatrade, 2026), not a quotation. Hertfordshire work generally runs below the London figures; your price follows a site visit. Prices checked September 2026. Full breakdown: Extension Costs & Planning.
Quick estimate for Letchworth Garden City
Letchworth Garden City range: choose above
The planning test your extension is judged against
Policy D2 of the adopted North Hertfordshire Local Plan grants permission for house extensions only where two tests are both met. The extension must be sympathetic to the existing house in height, form, proportions, roof type, window details, materials and the orientation of the main dwelling. It must also not dominate adjoining properties.
The local planning authority is North Hertfordshire District Council, working to the North Hertfordshire Local Plan 2011 to 2031, adopted on 8 November 2022. Those tests are design questions settled on the drawing, which is why the designer is appointed and the scheme resolved before anything is priced as a build.
Housing in Letchworth Garden City
Letchworth Garden City has around 15,290 homes on the Council Tax list: 67% post-war, 19% interwar and 14% predate 1919, with 22% flats and 78% houses. Extending a terrace in Letchworth Garden City means digging beside both neighbours, so two notices are normal, and 38% of homes here are terraced.
Heritage Foundation consent is a second application, and it is free if you apply first
Applying to the Heritage Foundation before doing the work is free. A retrospective application covering up to three alterations costs £216. The Foundation aims to process an application within six weeks once it has been registered and assigned to a case officer. Timescales extend where more information or changes are asked for, or where the case is referred to the Householders Applications Committee or the Board of Trustees.
It is worth being clear about where that control comes from. There is no Article 4 direction anywhere in Letchworth. North Hertfordshire has only two, both made on 9 October 2012, and they cover the Hitchin and Stockens Green conservation areas. The Letchworth constraint is the Foundation's, and it runs on its own timetable alongside the council's.
Which design principles apply depends on your character area
The Heritage Foundation splits Letchworth into Heritage and Modern character areas, each with its own Design Principles, and homeowners are asked to check which applies to their property before applying. That check belongs at the sketch stage, not after the drawings are finalised.
The Letchworth Conservation Area Appraisal describes off white and cream roughcast walling as the predominant walling finish within the conservation area. It also names garages or extensions enclosing the spaces between buildings as a change that harms the character of the Cheap Cottages Exhibition streets. A side infill on one of those streets is therefore a conversation to have early.
Flood zone and watercourses on the north west side
The Environment Agency Flood Map for Planning shows a single Flood Zone 3 corridor in Letchworth. It follows the watercourse valley in the north west of the town and runs north from roughly the latitude of the town centre. The rest of the built up area, including the whole southern half of the town, sits in Flood Zone 1.
The Statutory Main River Map records no main river within the built up area of Letchworth Garden City, so watercourses through the town are ordinary watercourses. That changes who you deal with about anything close to a channel. We check the zone for your address before a drainage strategy is drawn, because it can affect what the planning application has to include.
Kitchen extensions in Letchworth Garden City
49% of the houses in Letchworth Garden City are terraced. On a terrace the kitchen extension is built out from the back, into the side return where the house has one or across the full rear width, and a steel beam takes the old back wall so the kitchen and the new room become one space. The fit-out is priced on its own page, kitchen renovation in Letchworth Garden City; the build figures are in the table above.
Side return extensions in Letchworth Garden City
Side returns belong to Victorian and Edwardian terraces, and only 14% of homes in Letchworth Garden City predate 1919, so they are the exception here; most extensions run across the full rear width instead. Our side return extension page covers the build in detail, with build rates of £2,400 to £3,200 per m².
Double storey extensions in Letchworth Garden City
Under permitted development a two-storey rear extension can go no more than 3 metres beyond the original rear wall and must stay at least 7 metres from the rear boundary, and in a conservation area it is not permitted development at all. Many gardens are too short for the 7 metre rule, so a double storey extension in Letchworth Garden City often needs planning permission from North Hertfordshire District Council. On the 38% of homes that are terraced, the neighbours' windows usually decide the depth. Our double storey extension page explains the build, at £1,800 to £2,600 per m² across both floors.
- Planning applications and permitted development assessments
- Single and double storey extensions
- Full fit-out, finishing and making good
VOA Council Tax stock of properties, March 2025, for all 5 statistical areas in Letchworth Garden City. Counts rounded by the VOA.
Why Choose GS Renovation in Letchworth Garden City
An extension here runs two approval tracks at once, and they do not move at the same speed. We coordinate the structural engineer and the building control application and inspections, work alongside your designer on the planning application, then build the shell and the internal work with one team. That gives you one point of contact and one programme instead of a chain of separate trades. GS Renovation Ltd is a contractor: we appoint structural engineers and build to their calculations, and we never produce those calculations ourselves. Fully insured, registered in England, company number 14611254, and quotations are free and carry no obligation.
- Rated 4.9 from 31 reviews on MyBuilder
- Brick and mortar matched to the existing elevation
- Building control inspected at foundation, damp course and steel stages
Built a new extension at rear of property. Laid foundations, drainage, raised floor, new roof and completed all fixtures and interior decoration…
Very friendly and I am very pleased with the work he has done, also very clean worker and enthusiastic about his work. Can not fault this company!!!!…
Punctual, worked hard, and did exactly what he was asked and what he said he would do. Overall, a very high quality job. Will use again.
FAQ
Frequently Asked Questions about House Extensions Letchworth Garden City
Does the cost of house extensions rise in a 1950s to 1980s Letchworth Garden City property?
Age affects the cost of house extensions in Letchworth Garden City less than people fear; the structural part is broadly similar, within £2,000 to £2,800 per m² for a rear extension (London-derived). What differs in 1950s to 1980s housing is making good plaster, skirtings and floor finishes around the SG6 works, priced as separate lines. The sequence also holds: drainage under the footprint is diverted before foundations are poured.
What ground will the extension foundations be dug into?
In central, southern and south eastern Letchworth, the Lowestoft Formation, a chalky glacial till mapped as diamicton, over chalk bedrock. The British Geological Survey describes that formation as a sheet of chalky till with outwash sands and gravels, silts and clays, and a till matrix carbonate content of about 30 per cent. Along the far western edge no superficial deposit is mapped, so chalk lies at or near the surface. Trial holes settle the depth.
Where does the extension's foul drainage connect?
Into the existing house drainage, and the undertaker for waste water here is Anglian Water. The adopted North Hertfordshire Local Plan refers to capacity within the environmental consent at Letchworth Water Recycling Centre. The practical work is on your own plot: locating the run, checking the fall, and deciding whether the new connection is a branch or a new manhole. Where a drain passes under the extension footprint, the position has to be established before foundations are set out.
Is a rear extension permitted development in Letchworth, since there is no Article 4 direction here?
Permitted development rights are not withdrawn here, but permitted development is not the whole test. The Heritage Foundation's Scheme of Management is a legal agreement applying to all freehold properties within the Letchworth Garden City estate, and extensions sit on its published consent list, alongside conservatories, porches, garages and outbuildings. So a rear extension that needs no planning application can still need the Foundation's written consent before it is built. A listed building is the exception: work that affects its character, inside or out, including an extension or taking out a wall or chimney breast, needs listed building consent from North Hertfordshire District Council, and doing it without consent is a criminal offence.
What roof covering will suit the extension?
Plain clay tiles, on the appraisal's reading of the town. It records plain clay tiles as the predominant roofing material in the conservation area and says slate was avoided in the Garden City, so slates are not characteristic of it. For the Cheap Cottages Exhibition streets, the appraisal lists red brick, roughcast, and rendered cement panels or blocks painted off white or cream, with timber framing, hanging tiles and weatherboarding accentuating gables. Match the house, not the catalogue.
The garden falls away towards the fields at the north of the town. Where does the surface water go?
Onto your own plot, and it has to be shown. The adopted Local Plan records the land north of Letchworth as part of the higher northerly edge of the town. It falls north west towards Pix Brook and north and north east towards the Ivel Valley, with several ordinary watercourses draining that way. Where a soakaway is proposed, the chalky till mapped across the central and southern parts of the town decides whether it will work, so percolation testing comes first.
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