Home Extensions & Building Work: Costs, Planning & How to Get Quotes

A well-designed home extension can add significant value to your property — creating extra living space, improving functionality, and avoiding the cost and disruption of moving. But building projects are expensive, complex, and fraught with potential problems if you hire the wrong builder.

So how much should you expect to pay for an extension? Do you need planning permission? What qualifications should your builder have? And how do you avoid cowboys who take your deposit and disappear? This guide explains extension costs by type, planning and Building Regulations requirements, what to look for in a builder, and how SpecConnect can connect you with verified builders quickly.

How much does a home extension cost?

Extension costs vary enormously depending on size, specification, and location. Here are typical prices per square metre for 2026:

Single-storey rear extension

• Basic spec (brick, plaster, basic fixtures): £1,500-£2,000/m²
• Mid-range spec: £2,000-£2,500/m²
• High-end spec (bi-fold doors, underfloor heating, premium finishes): £2,500-£3,500/m²

Other extension types

• Two-storey extension (basic): £1,800-£2,300/m²; high-end: £2,800-£4,000/m²
• Loft conversion (basic dormer, ensuite): £30,000-£50,000
• Hip-to-gable loft conversion: £40,000-£65,000
• Full mansard loft: £50,000-£80,000+
• Side return extension (London terraces, typical kitchen): £35,000-£60,000
• Garage conversion (basic): £8,000-£15,000; full conversion: £15,000-£25,000
• London and South East: Add 20-30% to these prices

Costs typically include groundwork and foundations, brickwork and roof structure, windows and doors, first-fix electrics and plumbing, plastering, and basic heating. Architectural drawings (£1,500-£5,000), structural engineer fees (£500-£1,500), planning fees, and Building Control fees (£800-£1,500) are additional. Always clarify what's in scope before signing a contract.

Do you need planning permission?

Many home extensions fall under Permitted Development and do NOT require planning permission. Single-storey rear extensions on detached houses can extend up to 8 metres; semi-detached or terraced houses up to 6 metres. Two-storey extensions usually require planning permission. Loft conversions are Permitted Development if the volume added is under 40m³ (terraced) or 50m³ (detached/semi).

You WILL need planning permission if your property is in a conservation area or is listed, you've already used Permitted Development rights, the extension exceeds limits, or it faces the street. Always check with your local council before proceeding — many offer a free pre-application advice service.

Building Regulations approval is ALWAYS required

Even if you don't need planning permission, you MUST comply with Building Regulations for any structural work. This covers Part A (structure), Part B (fire safety), Part C (resistance to moisture), Part E (sound insulation), Part F (ventilation), Part L (energy efficiency), Part M (access), and Part P (electrical safety). Your builder should handle Building Control notifications and inspections. You'll receive a Completion Certificate at the end — essential for selling your property.

What qualifications and insurance should a builder have?

• Public liability insurance (minimum £5 million)
• Employer's liability insurance (if they have employees)
• FMB (Federation of Master Builders) membership
• Trustmark or Checkatrade approved
• NHBC registration (for new builds and major extensions)
• Portfolio and references — photos of completed extensions and testimonials
• Building Control experience — familiar with submitting plans and managing inspections

How long does an extension take?

• Single-storey rear extension (basic 20-30m²): 8-12 weeks
• Two-storey extension: 12-20 weeks
• Loft conversion: 6-10 weeks
• Side return extension: 8-12 weeks
• Garage conversion: 4-8 weeks

Weather, material delays, and Building Control inspections can extend timescales. Always build in a 10-20% buffer.

Party Wall Act: when do you need to notify neighbours?

The Party Wall etc. Act 1996 requires you to serve notice on neighbours if you're building on or near a party wall, excavating within 3 metres of a neighbour's structure (if going deeper than their foundations), or excavating within 6 metres (if going deeper than a line drawn at 45° from their foundation). Notices must be served 2 months before work starts. Party Wall surveyor fees: £700-£1,500 per property if disputes arise.

Common extension problems and how to avoid them

• Builder goes bust mid-project: check financial stability (Companies House), use insurance-backed warranties, pay in stages
• Project overruns budget by 20-50%: get detailed written quote, agree change process in writing, build 10-15% contingency
• Building Control rejects work: hire builder with Building Control experience, attend inspections yourself
• Structural issues (cracks, subsidence): always use a structural engineer for foundation design, conduct soil surveys if needed

How to avoid cowboy builders

• ❌ Demands large upfront payment (more than 20% is a red flag)
• ❌ No written quote or contract
• ❌ Can't provide insurance proof or references
• ❌ Only accepts cash
• ✅ Get 3 written quotes
• ✅ Check reviews (Google, Checkatrade, Trustpilot)
• ✅ Ask for references and visit completed projects if possible
• ✅ Pay in stages tied to milestones (5-10% retention at completion)
• ✅ Use a contract (FMB or JCT standard contracts)
• ✅ Use verified platforms like SpecConnect where builders are checked before joining

How SpecConnect connects you with verified builders

Finding a reliable builder usually means weeks of phone calls, vague quotes, waiting for site visits, and chasing responses. SpecConnect removes that friction. You upload details of your extension project — photos of the property, measurements, your specification. The app shares these with verified builders covering your area. They can assess the work before responding, which means more accurate quotes without wasted site visits.

Most SpecConnect users receive competitive extension quotes within a few hours. SpecConnect is free to use for homeowners and property developers. It covers 35+ trade categories beyond building — electricians, plumbers, roofers, landscapers, and more.