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SIPs Extension

How big, and how well finished? Set both and see what a SIPs extension costs before you speak to anyone.

24 m²
Finish

Aluminium sliding or bifold doors, two roof lights, engineered timber floor, underfloor heating

Typical cost, built and finished

£41,000 to £50,500

£1,700 to £2,100 per m², finished, including VAT. A guide, not a quote.

Cross-section through a SIPs extension wallFrom outside to inside: 20 mm cladding, a 25 mm drained cavity, a 142 mm structural insulated panel made of two 15 mm OSB skins and a 112 mm rigid foam core, a 25 mm service void and 12.5 mm plasterboard.OutsideRoom20 mmCladding25 mmDrained cavity15 mmOSB/3 skin112 mmRigid foam corePUR or PIR here; EPS panels are thicker15 mmOSB/3 skin25 mmService void12.5 mmPlasterboard142 mm panel
  1. 20 mmCladding
  2. 25 mmDrained cavity
  3. 15 mmOSB/3 skin
  4. 112 mmRigid foam core
  5. 15 mmOSB/3 skin
  6. 25 mmService void
  7. 12.5 mmPlasterboard
A typical SIPs wall, drawn to scale: about 225 mm overall.

What a SIP wall is made of

Two sheets of OSB bonded to a rigid foam core. One panel carries the roof, holds the heat in and arrives cut to your drawings, so walls go up in days. At 142 mm, with an insulated lining, it meets the U-value below.

  • 0.18 W/m²K

    Limit for a new extension wall in England.

  • 0.15 W/m²K

    Limit for its roof.

  • 1.4 W/m²K

    Limit for windows and glazed doors.

Where these figures come from
A zinc-clad glazed extension with its doors open to a lawn, on the back of a modern rendered house

What a SIPs extension costs

Built and finished, budget £1,300 to £2,400 per m² of floor including VAT. A typical national range for 2026; London and the South East usually cost 20 to 40% more. A guide, not a quote.

Floor areaStandard finishHigh spec
15 m²£19,500 to £25,500£31,500 to £36,000
20 m²£26,000 to £34,000£42,000 to £48,000
30 m²£39,000 to £51,000£63,000 to £72,000
40 m²£52,000 to £68,000£84,000 to £96,000

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Four parts of the job, in detail

OSB-faced wall panels of a new timber building standing against a blue sky

Planning and Building Regulations

In England, a single-storey rear extension up to 4 m deep on a detached house, or 3 m on any other, usually needs no planning application. One condition catches SIPs: the outside must look similar to the house, so timber cladding on a brick home means applying.

Building Regulations approval is always needed. Scotland has its own limits and requires a building warrant first.

Official guidance

From drawings to a dry shell

  1. Drawings and engineering

    3 to 6 weeks

    Plans become panel drawings and calculations.

  2. Base

    1 to 2 weeks

    Foundations and a level slab.

  3. Panels cut

    2 to 4 weeks

    Factory-made while concrete cures.

  4. Panels up

    3 to 5 days

    Walls and roof stood and wrapped.

  5. Finish

    4 to 6 weeks

    Cladding, glazing, wiring and plaster.

Questions we get asked

Is a SIPs extension cheaper than brick and block?
Not by much. A finished SIPs extension typically costs £1,300 to £2,400 per m² and brick and block £1,800 to £3,000, both including VAT, and the ranges overlap. The clearer gains are weeks saved and heat kept.
Do SIPs need special foundations?
No. Strip, raft or screw piles all work; depth follows your soil. Foundations in detail.
Can I build one from a kit?
Yes, though most buyers pay the supplier to stand the shell. How kits work.
How long will it last?
Certified panel systems are assessed for 60 years or more, provided the OSB stays dry.

Get a quote for your SIPs extension

Postcode, four quick questions about the build, then how to reach you. It takes about a minute and costs nothing.

We match homeowners with panel firms, free of charge, and do not build extensions ourselves. We look for a company that designs and erects SIPs extensions near your postcode and tell you its name. Your details go no further until you say yes.

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