How much does a rear extension cost per square metre in West Sussex?

By James Reilly, Swanbourne Construction Ltd · Published 15 October 2026 · National cost data, fees and Southern Water charges checked October 2026

A single-storey rear extension around Bognor Regis and Chichester typically costs £2,400 to £3,600 per square metre including VAT — £2,000 to £3,000 before VAT. So a 20m² extension is roughly £48,000 to £72,000 and a 50m² one £120,000 to £180,000, including VAT. That buys a finished, plastered, heated room with its foundations, roof, glazing and the opening into the house. It does not usually include a kitchen, professional and council fees, a party wall surveyor or a Southern Water build-over agreement, which together can add a third again.

We are building a 50m² rear extension at 29 Victoria Road in Bognor Regis as this is written. The contract is £120,000 including VAT — £2,400 per square metre, the bottom of our band, on a well-specified job. This guide uses it to show what a per-square-metre rate does and does not mean.

Our band, and how it compares

Two of those sources add that London and the South East typically cost 25–35% more (Homebuilding & Renovating) or 30–50% more (FMB) than the national average. Put that uplift on the national figures and our band sits in line with, or slightly below, what you would expect on the West Sussex coast. The important point is the VAT column. Online figures are almost always before VAT; a homeowner pays it and cannot claim it back, so ours include it.

A real example: 50m² in Bognor Regis

The Victoria Road extension is a 50m² single-storey rear extension opened right through into the existing house. The £120,000 contract, including VAT, pays for:

• foundations, the shell, a fibreglass flat roof and two roof lanterns;
• bifold doors to the garden, new windows and a new back door;
• the old rear wall taken out completely, with the structure to carry the house above;
• underfloor heating under a tiled floor right across the ground floor;
• high-specification LED lighting throughout;
• a separate boiler room built at the side, so the plant can be serviced without lifting a heated floor.

That is £2,400 per m² — the bottom of the band, not the middle — on a job with plenty in it. Swap the underfloor heating for radiators, the fibreglass for felt and the lanterns for nothing, and the rate falls. Put the same footprint on a sloping plot with soft ground and it rises. Specification moves the money far more than floor area does, which is why anybody quoting a rate per square metre before asking what you want in the room is guessing.

What moves the rate up or down

Two other things are worth knowing. Small extensions cost more per square metre, because scaffold, foundations, steelwork and setting up the site do not shrink with the floor area. And the ground decides more than the drawing. On the coastal plain around Bognor Regis, Pagham and Aldwick, soft clays, silt and high groundwater are common, and foundations are priced against an assumed depth that the trial holes then confirm. See building near the West Sussex coast.

What the rate usually leaves out

Fee sources: MHCLG planning fees from 1 April 2026; Southern Water developer services fees 2026/27; national cost guides for professional fees. Draft regulations laid before Parliament in July 2026 would raise the householder fee to £575 and prior approval to £310 from 8 December 2026 if approved, so check the current figure.

Our quotations list every one of these as an exclusion so nobody is surprised. The engineer and the council invoice you directly.

Building over a sewer: the cost nobody mentions

If any part of your extension or its foundations comes within 3m of a public sewer, you need Southern Water's approval before you build. That catches far more people than expect it, for one reason: on 1 October 2011, shared drains that served more than one property were transferred to the water companies. The shared pipe running across the backs of a row of semis or terraces in Bognor Regis or Littlehampton is very often now a public sewer, on private land, and frequently not on any map.

• Only smaller, shallower sewers — 225mm diameter or less and under 3m deep — can normally be built over.
• A manhole cannot end up inside the extension; it has to be moved, which is extra work.
• Foundations within 3m must go below the level of the sewer.
• Southern Water's 2026/27 fee for a formal build-over application is £771.60 including VAT; a lower-cost declaration route exists for simpler cases.

It is checked at the site visit — we lift the manhole covers — and planned into the programme rather than discovered by the digger.

Glazing: the 25% rule and what it costs

Big glass is the reason most people want an extension, and it is also the item that moves the price fastest. National guides put a set of aluminium bifolds at roughly £3,000 to £7,000 supplied and fitted depending on width, and a medium aluminium roof lantern at around £2,000. Specification matters: thermal performance, the colour, the threshold, whether the doors fold or slide.

The energy rules matter too. Approved Document L limits glazing in an extension — windows, doors and rooflights together — to around 25% of the new floor area plus any openings lost from the house, unless a whole-house calculation shows more is acceptable. A wall of bifolds and two lanterns can exceed that, which is fine if the calculation is done at design stage and expensive if it is discovered at Building Control. New windows must meet a U-value of 1.4 and rooflights 2.2. These rules stay in force until the Future Homes Standard documents take effect on 24 March 2027.

VAT on extensions

Building an extension to an existing house is standard-rated at 20%. There are narrow exceptions — certain adaptations for disabled people, and some energy-saving materials, which are zero-rated until 31 March 2027 — but they are exceptions. If one quotation is "plus VAT" and another includes it, add 20% to the first before you compare. Our guide to comparing builders' quotes goes through the rest.

Three quick worked examples

On our price guide, a large single-storey extension is banded at £75,600 to £120,000 including VAT, and the bottom of that range is a straightforward rectangle on decent ground with a flat roof and standard windows. That is a starting point, not a quotation; a free site visit turns it into a number.

Questions people ask about extension costs

Where these facts come from

• Checkatrade: house extension cost (2026).
• Homebuilding & Renovating: how much does an extension cost (February 2026).
• Federation of Master Builders: single-storey extension costs (March 2026).
• Southern Water: building over or near a sewer, guidance notes.
• MHCLG: planning fees from 1 April 2026.
• Approved Document L: conservation of fuel and power.