How Much Will My Extension Cost? Understanding Builder's Estimates and the Path to Accuracy
Planning a 3x7m extension? Learn why builder estimates vary from Google searches, what drives costs from £2,500-£3,500/m², and how to get accurate quotes with proper specifications.
Quick Summary
Planning a rear kitchen extension? Here's what you need to know about costs:
- Google estimates: £2,500-£3,500 per m² (rough guide only)
- 3x7m extension: Approximately £63,000 at £3,000/m²
- Reality: Initial estimates often miss key costs like kitchens (£21k+) and electrics (£8k+)
- Solution: Detailed specifications = accurate quotes
Want to calculate your extension cost accurately? Download our free Extension Cost Calculator to understand the real numbers.
The Big Question: How Much Will This Cost?
So, you're planning a rear kitchen extension — maybe something like 3 by 7 metres — and the big question on your mind is: How much is this going to cost?
You're not alone. Most homeowners want cost clarity from the outset. But getting an accurate estimate isn't as simple as punching numbers into Google — and that's where expectations often clash with reality.
Route 1: The Digital Shortcut (Google & AI Apps)
In today's digital world, you might type "3x7m extension cost" into Google or ask an AI-powered app. You'll likely see a broad range — anywhere from £2,500 to £3,500 per square metre.
Using a mid-range figure of £3,000/m², a 21m² extension would come in at around £63,000. It sounds like a useful benchmark — and it is — but it's only a rough guide and far from a tailored quote.
Why Online Estimates Fall Short
- Based on historical averages across different regions
- Don't account for your specific site conditions
- Exclude hidden costs like structural work or drainage
- Can't factor in your finish quality preferences
Route 2: The Traditional Approach (Ask a Builder)
Many still take the traditional path: call a builder and ask, "Roughly how much would this cost?"
You'll often hear back: "Do you have any drawings?"
Without drawings, a builder can only offer a ballpark estimate — a figure based on general experience and rules of thumb. Something like:
"Three by seven metres? About £3,000 per m² — so say, £63,000, plus or minus depending on your kitchen, structure, and internals."
It's a starting point — not a fixed price — and it comes with a big health warning.
Why It's Just an Estimate
Builders can't (and shouldn't) give fixed prices without detail. There are too many variables:
- Structural changes and demolitions
- Internal reconfigurations
- Kitchen specification and appliances
- Electrical and plumbing requirements
- Foundation conditions
- Access constraints
And because builders don't want to scare you off before you're committed, many will understate the true cost or leave out key components.
The Path to Pricing Accuracy: Levels of Detail
Want to move from guesswork to meaningful figures? You'll need to work through these layers of detail:
Level 1: Planning Drawings (30-40% Accuracy)
Planning drawings give a clearer picture of intent, but still only offer indicative pricing. There are typically only 4 or 5 drawings required for planning approval.
What's missing? Internal layouts, structural details, finishes, services specification.
Level 2: Building Control Drawings (60-70% Accuracy)
Also include structural calculations and construction details, which help improve cost accuracy to around 60–70%.
However, they usually exclude:
- Kitchen fittings and appliances
- Power, lighting and heating layouts
- Interior finishes and fixtures
- Specific product specifications
Level 3: Full Specification of Works (85-90% Accuracy)
This is the critical piece. It details the materials, finishes, systems, and methods — enabling builders to cost accurately and compare like for like.
A proper specification includes:
- All building materials and suppliers
- Complete electrical and plumbing layouts
- Heating system specification
- Window and door schedules
- Flooring and tiling details
- Paint colours and finishes
Level 4: Final Specification with Interior Design Package (95%+ Accuracy)
Once every tap, tile, and socket is defined, your builder can quote with confidence. This includes:
- Full kitchen design with appliance models
- Complete lighting schedule
- Sanitaryware specifications
- Ironmongery (handles, hinges, locks)
- Final finishes and colours
The Hidden Cost Trap: Understanding Provisional Sums
When details aren't finalised, builders include provisional sums — placeholder budgets for items like kitchens and electrics.
This is where budget shock happens.
Example 1: Kitchen Installation
A realistic kitchen budget for a modern extension:
| Item | Cost |
|---|---|
| Appliances (oven, hob, extractor, dishwasher, fridge) | £5,000 |
| Base and wall units | £10,000 |
| Granite or composite worktops | £3,500 |
| Professional installation | £2,500 |
| Total | £21,000 |
But if the builder's provisional sum is only £15,000, and that sounds totally reasonable when you're budgeting — you're heading toward a £6,000 shortfall.
Example 2: Electrical Installation
Your builder suggests allowing for 50 power and lighting points. Sounds generous, right?
Wrong.
A modern open-plan kitchen-diner in a three-bedroom semi-detached house typically needs:
| Area | Points Required |
|---|---|
| Kitchen sockets (appliances, prep areas) | 20-25 |
| Kitchen lighting (ambient, task, accent) | 8-12 |
| Dining area (sockets + lighting) | 10-15 |
| General room sockets and switches | 15-20 |
| Realistic total | 80+ points |
At £100 per point, that's an £8,000 electrical cost, not £5,000.
If provisional sums don't account for realistic requirements, a £3,000+ overspend creeps in fast.
Reality Check: An Estimate Is Not a Quote
Most projects go over budget because clients request prices without providing the detail needed to price them accurately.
Builders operate commercially — they must protect their time and risk. They'll give you a number to keep you engaged, but it's inherently cautious and often incomplete.
The only way to get a realistic, reliable price is to provide a complete design and specification.
Our Top Tip: Follow a Structured Process
If you want clarity, control and confidence, follow this proven process:
- Start with design → Understand what's possible
- Gain approvals → Secure planning and building control
- Create a full specification → Define every detail
- Request quotes → Get accurate, comparable pricing
That's why our design and build service integrates all these steps — so you get accurate costs and a clear path forward from the outset.
Frequently Asked Questions
How accurate are online extension cost calculators?
Online calculators provide rough estimates based on averages (typically £2,500-£3,500/m²). They're useful for initial budgeting but can't account for your specific site conditions, structural requirements, or finish quality. Expect them to be within ±20-30% of the actual cost.
Why do builder estimates vary so much?
Builder estimates vary because they're working with different assumptions about:
- What's included in the scope (demolition, drainage, finishes)
- Quality of materials and fixtures
- Site access and working conditions
- Your specific structural requirements
- Kitchen and electrical specifications
Without detailed drawings and specifications, builders are essentially guessing different things.
When should I get a full specification?
Get a full specification before requesting formal quotes. This typically happens after:
- Planning approval (if required)
- Building control drawings are complete
- You've made decisions about kitchen, bathroom, flooring, etc.
This ensures all builders are quoting for exactly the same work.
What's the difference between an estimate and a quote?
- Estimate: An educated guess based on limited information. Can change significantly as details emerge.
- Quote: A fixed price based on detailed specifications. Should only vary if you change the scope.
Always clarify which you're receiving. Many builders call their pricing "quotes" when they're actually estimates.
How much should I budget for unexpected costs?
Add a 10-15% contingency to your builder's quote. This covers:
- Unforeseen structural issues
- Ground condition problems
- Changes to regulations during the project
- Small design changes you make during construction
On a £63,000 extension, that's £6,300-£9,450 contingency.
Can I save money by doing some of the work myself?
Generally not recommended for structural extensions. Building control requires qualified professionals for:
- Structural work
- Gas and electrical installation
- Drainage and plumbing
You might save money on decorating, landscaping, or project management — but factor in the time cost and potential delays.
Download Our Free Extension Cost Calculator
Get accurate cost estimates for your extension project with our comprehensive calculator. Includes:
✓ Detailed cost breakdown by trade ✓ Kitchen and electrical point calculators ✓ Material specification checklist ✓ Provisional sum warning guide ✓ Contingency planning worksheet
Download the Extension Cost Calculator (PDF)
Or book a free consultation and we'll help you calculate your specific project costs with our 37 years of expertise.
Take the Next Step
Understanding extension costs is just the beginning. The real value comes from working with professionals who can guide you through the entire process — from initial concept to final handover.
Our design and build service includes:
- Accurate cost estimates from day one
- Full architectural and structural design
- All planning and building control applications
- Project management and construction
- Fixed-price contracts (no hidden costs)
Ready to get started? Book your free consultation today and let's discuss your project with complete cost transparency.
With 37 years of experience in residential extensions across London and surrounding areas, Ashton Paul has helped hundreds of homeowners navigate the extension process with confidence. From initial budgeting to final handover, we're here to guide you every step of the way.