1. How much does a house extension cost?

Short answer: it depends. Long answer: let’s break it down properly.

– Basic rear extension – £1,800–£2,500 per sqm
– Mid‑to‑high spec finish – £2,500–£3,500+ per sqm

– Loft conversion – £40,000–£75,000+

– Side return – £50,000–£90,000

These prices are for build only – they do not include kitchen, finishes, or professional fees. Remember as well that sometimes there will be VAT is on top of contractor quotes – that’s an instant +20%. If it’s not disclosed, always ask if they are VAT registered and if VAT is required on top.

Add‑on costs to factor in:

– Architect or designer: ~7‑15% of total build cost
– Structural engineer: £1,000–£3,000 (depending on complexity)
– Party Wall Surveyor: £900–£1,500 per neighbour
– Planning application fee: ~£238 (if needed)
– Building Regs drawings & submission: ~£1,000–£3,000
– Kitchen/bathroom install: easily £10k–£50k+
– Contingency: ALWAYS add 10‑15%

Based on construction costs of London & SE average. 

2. What permissions do I need?

Here’s where it gets spicy. In the UK, you may not need full planning permission thanks to something called Permitted Development Rights (PD) – but there are rules.

What’s covered by PD?

• Single-storey rear extension (up to 3m on terrace/semi, 4m on detached)
  
• Loft conversions with rear dormer
  
• Side extensions (single-storey, up to 3m wide)

BUT… you must meet specific criteria (height, materials, etc.). And PD doesn’t apply if:

• You’re in a conservation area, AONB, or a flat
  
• Your property has had its PD rights removed
  
• You’re extending forward or increasing roof height

Planning permission required if:

• You’re going outside PD limits
  
• Changing the front façade
  
• Extending beyond half the width of the house
• Adding two storeys

Building Regulations:

This is mandatory no matter what. It ensures your build is structurally sound, safe, and up to code (think: insulation, fire safety, drainage, electrics). You’ll either submit:

• Full Plans Application – ideal for complex jobs
  
• Building Notice – quicker, for smaller works

Party Wall Agreement:

If you’re building on or near a shared boundary, you legally need to notify your neighbours. If they don’t agree, you’ll need a Party Wall Surveyor to mediate.

💡 Top tip: Don’t rely on your builder to “just sort it”. You’re legally responsible for permissions and notices.

4. How long does it all take?

💡 Top tip: Always allow a few weeks buffer. Bad weather, delays with materials (hello tiles from Spain), and scope creep all cause setbacks. Take it from us! 

7. When should I get an interior designer involved?

Honestly? From the beginning. An architect will design the structure – but an interior designer will make sure it functions for real life. Think:

• Where your furniture will actually go with a carefully considered structure
  
• Where to place lighting circuits and sockets
  
• How to get a kitchen layout that doesn’t suck
  
• Making your Pinterest board a reality without wasting ££ on bad purchases

💡 Top tip: If you’re spending £100k+ on a build, don’t wing the interiors. It deserves just as much planning.

Extending your home is one of the biggest investments you’ll make. Done right, it adds value, function, and a whole new lease of life to your property. But the trick is planning properly, getting the right team in early, and avoiding common pitfalls.

Want help planning your extension?

If you’re feeling overwhelmed, unsure how to lay out your new space, or just need some creative ideas to bring it to life – we’re here for that.

👉 Contact us here