Homeowners often think extension costs are dominated by materials. Wrong. They think brickwork costs more than plumbing. Wrong again. The way budgets actually break down surprises most people. If you understand where money goes, you can make smarter decisions about where to save and where to spend. A thousand pounds saved on finishing materials might mean a thousand pounds worse quality. A thousand pounds saved on structural coordination might mean building control failures and £5,000 in fixes. This is the real breakdown.
In this article
- The Master Budget Breakdown: Every Line Item
- Design and Planning Fees: What They Actually Cost
- Planning Application Fees: What You Pay the Council
- Rear Extension Cost Example (Single Storey, £60k)
- Side Return Extension Cost Example (Two Storey, £120k)
- Loft Conversion Cost Example (£80k)
- Where Homeowners Overspend (And Why)
- Where Legitimate Savings Are Actually Possible
- FAQ
The Master Budget Breakdown: Every Line Item
For a typical London extension (West London: Ealing, Harrow, Hillingdon, Hounslow; North: Barnet, Brent), here’s where your budget goes, expressed as a percentage of total cost:
| Item | Percentage | Notes |
|---|---|---|
| Design and Planning | 5 to 8% | Design fee, planning application, building control. The design fee itself is usually a small slice of this, see below for real numbers |
| Demolition and Prep | 4 to 6% | Removing existing structures, site clearance, asbestos testing |
| Groundworks and Foundations | 8 to 12% | Digging, concrete, drainage (varies by soil type) |
| Structural Frame | 15 to 20% | Walls, roof, columns, lintels, structural steel |
| External Envelope | 12 to 16% | Brickwork, render, windows, doors, roof covering |
| Mechanical Systems | 8 to 12% | Plumbing, heating, ventilation, air tightness |
| Electrical and Data | 6 to 8% | Wiring, circuits, lighting, smart home prep |
| Interior Finishes | 12 to 18% | Plasterboard, flooring, internal walls, painting |
| Kitchen and Bathroom | 6 to 12% | If included; can vary wildly |
| Contingency | 5 to 10% | Unexpected findings, changes, delays |
Design and Planning Fees: What They Actually Cost
The figures below are the fees we typically quote for design, survey, and planning work on residential extensions and conversions. They cover a full measured site survey, existing and proposed drawings, and submitting and managing the planning application.
| Project type | Example design and planning fee |
|---|---|
| Single storey rear extension | £800 to £1,200 |
| Single storey side extension | £800 to £1,200 |
| Single storey side and rear extension | £1,000 to £1,500 |
| Two storey rear extension | £1,000 to £1,500 |
| Two storey side extension | £1,000 to £1,500 |
| Two storey side to rear extension (wraparound) | £1,200 to £1,800 |
| Loft conversion | £1,000 |
These are example fees for typical projects, not a fixed quote. Your actual fee depends on the property, site constraints, and exact scope. Get a fixed price for your specific project before committing.
Tell us the scope and we’ll quote a fixed design and planning fee before any work starts, no surprises later.
Get a fixed quote →Planning Application Fees: What You Pay the Council
Separate from your design fee, the council charges its own fee to process a planning application. These are national default fees set by government; some councils can vary slightly, so always confirm with your local planning authority before applying.
| Application type | Fee now | Fee from 8 December 2026 |
|---|---|---|
| Householder application (extension or alteration to a house) | £548 | £575 |
| Full application, new dwelling (under 10 units) | £610 per dwelling | £752 per dwelling |
| Prior approval, larger home extension (exceeding standard permitted development limits) | £249 | £310 |
| Certificate of lawful development, existing use or development | Same as the equivalent planning application fee | Same as the equivalent planning application fee |
| Certificate of lawful development, proposed use or development | Half the equivalent planning application fee | Half the equivalent planning application fee |
Fees shown are national default rates for England as of 2026. Not sure which application type your project needs, permitted development, prior approval, or a full householder application? See our guide to what’s actually lawful under permitted development in 2026.
Rear Extension Cost Example: Single Storey, £60,000
Let’s break down a realistic single storey rear extension (approximately 20 sqm, around £3,000 per sqm):
| Item | Amount | % | Notes |
|---|---|---|---|
| Design and Planning | £800 to £1,200 | 1 to 2% | Design fee only, see above for council and statutory fees |
| Demolition and Site Prep | £3,500 | 6% | Removing existing structures, site setup |
| Foundations | £6,000 | 10% | Concrete, drainage connections, ground conditions |
| Brickwork and External | £13,000 | 22% | Matching existing brick, windows, door, roof |
| Plumbing | £5,200 | 9% | Waste, water, drainage connections |
| Heating | £3,500 | 6% | Extending radiators or underfloor heat |
| Electrical | £5,200 | 9% | Circuits, outlets, lighting |
| Plasterboard and Finish | £8,600 | 14% | Walls, ceiling, taping, painting, skirting |
| Flooring | £5,200 | 9% | Screeded concrete, finished flooring |
| Doors and Internal Fittings | £3,500 | 6% | Internal door, shelving, mirrors |
| Contingency | £5,300 | 9% | Unexpected soil, changes, additions |
| TOTAL | £60,000 | 100% |
What this actually looks like: a new kitchen diner space. It’s bright, connected to the garden, but straightforward. No major structural surprises. Decent materials. This timeline is approximately 4 to 5 months from starting design.
Biggest cost driver: structural frame and external envelope (brick, windows, roof) together account for around 30% of cost. Many people think it’s lower.
Side Return Extension Cost Example: Two Storey, £120,000
A two storey side return adds complexity. More structural work, more height coordination, more intricate detailing where it meets the existing house.
Budget: approximately 35 to 40 sqm, around £3,000 to £3,500 per sqm
| Item | Amount | % |
|---|---|---|
| Design and Planning | £1,000 to £1,500 | 1% |
| Demolition and Site Prep | £3,600 | 3% |
| Groundworks and Foundations | £10,800 | 9% |
| Structural Frame (steel and brick) | £26,400 | 22% |
| Brickwork and External | £18,000 | 15% |
| Windows and Doors | £6,000 | 5% |
| Roofline and Integration | £4,800 | 4% |
| Plumbing | £6,000 | 5% |
| Heating | £4,800 | 4% |
| Electrical | £7,200 | 6% |
| Plasterboard and Finish | £9,600 | 8% |
| Flooring | £4,800 | 4% |
| Contingency | £16,750 | 14% |
| TOTAL | £120,000 | 100% |
What this actually looks like: an extra bedroom and ensuite on the first floor, plus a spacious living area below. This is substantial. Timeline is typically 6 to 7 months from starting design.
Key difference from the rear extension example: structural costs jump to 22%, and contingency is larger because multi storey complexity means more things can surprise you.
Loft Conversion Cost Example: £80,000
Lofts are faster to build than ground floor extensions (no new foundations) but still intricate.
Budget: approximately 25 to 30 sqm, around £2,700 to £3,200 per sqm
| Item | Amount | % |
|---|---|---|
| Design and Planning | £1,000 | 1% |
| Existing Structure Inspection | £1,400 | 2% |
| Roof Removal and Prep | £3,700 | 5% |
| New Roof Structure | £13,900 | 17% |
| Structural Steel | £7,400 | 9% |
| Windows, Skylights and Dormers | £9,300 | 12% |
| Insulation and Air Tightness | £5,600 | 7% |
| Plumbing | £3,700 | 5% |
| Electrical | £5,600 | 7% |
| Plasterboard and Finish | £10,200 | 13% |
| Flooring | £3,700 | 5% |
| Bathroom Suite (if included) | £7,400 | 9% |
| Contingency | £7,100 | 9% |
| TOTAL | £80,000 | 100% |
What this actually looks like: a primary bedroom with ensuite, or two additional bedrooms. Loft conversions feel luxurious because you’re gaining a whole floor. Timeline is typically 4 to 5 months from starting design.
Key difference from ground floor extensions: no foundations, which saves 8 to 12% of budget elsewhere, but insulation and air tightness are critical here and roof work is weather dependent.
Where Homeowners Overspend (And Why)
1. Kitchen and Bathroom Finishes
Many people budget £4,000 for a kitchen in their extension, thinking that’s plenty. Then they visit showrooms and suddenly it’s £8,000 to £12,000. Kitchens and bathrooms can consume 15 to 25% of your total budget if you’re not careful. A basic fitted kitchen is £4,000 to £6,000. A good one is £8,000 to £12,000. A showroom kitchen is £15,000 or more. Decide your threshold early and stick to it.
2. Premium Finishes Throughout
Choosing premium flooring, paint, and fixtures in every room adds up quickly. A premium floor at £80 per sqm instead of standard at £40 per sqm costs an extra £800 to £1,600 on a 20 sqm extension. You don’t need the same finishes everywhere. High traffic areas justify premium finishes. Less used spaces can be standard.
3. Structural Surprises (Contingency Blowout)
Many people budget 5% contingency. That’s not enough for older properties. Lofts with unknown roof conditions, properties with subsidence history, or anything listed should budget 10 to 15%. A structural surprise can cost £2,000 to £5,000 to resolve. Build your contingency accordingly.
4. Scope Creep
“While we’re building, let’s also…” Every added feature, an extra bathroom, upgraded heating, additional electrical circuits, adds 2 to 5% to the budget. Adding three things adds 6 to 15%. Lock your scope before starting construction. Changes mid build cost significantly more.
Where Legitimate Savings Are Actually Possible
1. Simple Design (Save 2 to 3%)
A straightforward rear extension costs less than a complex hybrid design requiring multiple roof angles. Simple geometry is inherently cheaper.
2. Standard Materials (Save 5 to 8%)
Matching existing brick costs more than using standard render. Standard windows cost less than heritage style windows. If you’re not constrained by planning, standard materials save real money.
3. Integrated Kitchen Later (Save 5 to 10%)
Build the shell now, install the kitchen in 6 months. Labour costs less when not rushed. You’re also not paying a premium for installation coordination during construction.
4. Efficient Contractor Selection (Save 3 to 5%)
Three competitive quotes will show you the range. A contractor willing to work efficiently for a slightly lower margin costs less than one padding every estimate.
5. Phased Finishes (Save 5 to 8%)
Paint and flooring are moveable costs. Do them yourself or phase them after construction if budget is tight.
6. Standard Contingency vs Unnecessary Padding (Save 5 to 10%)
A 5% contingency is realistic for straightforward projects. A 15% contingency is padding. Know the difference for your project type.
Important: don’t cut structural, insulation, or plumbing budgets. These savings haunt you later.
We’ll review your scope, location, and property type to give you a budget range and show you where your money goes.
Get in Touch →For projects over £100k, we coordinate with a QS to ensure your budget stays on track and contractors aren’t padding estimates.
Enquire →The Real Budget Picture
A typical London extension costs £2,800 to £3,500 per square metre, depending on complexity and finishes. Costs vary slightly across London boroughs (West London boroughs like Ealing and Harrow, North London including Barnet and Brent).
- Rear extension, 20 sqm: £56,000 to £70,000
- Side return, 35 sqm: £98,000 to £122,500
- Loft conversion, 25 sqm: £70,000 to £87,500
Structural work (frame, external envelope, foundations) accounts for 40 to 45% of your budget. Finishes and systems (plumbing, electrical, interior) are 35 to 40%. Design and contingency are 15 to 20%.
The difference between budget and reality usually comes from scope creep or contingency underestimation. Neither is a problem if you plan for it upfront.
Frequently Asked Questions
Why is structural steel so expensive?
Structural steel is engineered specifically for your building. It’s designed, manufactured to tolerances, delivered, and installed by certified workers. It’s not a commodity item. A single structural beam can cost £2,000 to £4,000.
How much does design and planning actually cost?
For a typical single storey extension, £800 to £1,200. Two storey extensions and combined schemes run £1,000 to £1,800. Loft conversions are typically £1,000. This covers the survey, drawings, and planning application, not the council’s own application fee or building regulations and structural engineer costs, which are separate.
What planning application fees will I pay the council?
A householder application (a standard extension) currently costs £548, rising to £575 from 8 December 2026. Prior approval for a larger single storey rear extension is £249, rising to £310. These are national default fees; your council may vary slightly.
Can I build the shell now and finish later to spread costs?
Yes, and it’s smart. You can build the structure and external envelope, then pause. Interior finishes can happen 6 months later when you have more budget. Phase 1 might be 60% of cost, phase 2 the remaining 40%.
What’s included in “contingency”?
Unforeseen structural issues, soil conditions requiring deeper foundations, asbestos findings, building control requests for additional work, material price increases, weather delays, and local authority requirement changes. Basically, anything not on the original plan.
Do I need to hire a quantity surveyor?
For projects over £100,000, yes. A QS costs £1,500 to £3,000 but catches cost overruns and ensures contractors aren’t padding estimates. They pay for themselves.
Why does plumbing cost so much?
Plumbing involves specialists, materials that must meet building code, potential modifications to existing systems, and testing and certification. A bathroom with new waste connections to mains can easily cost £3,000 to £6,000.
Is it cheaper to hire a builder or project manage contractors separately?
A single builder handles everything, which is easier to manage with a single point of responsibility. Hiring contractors separately requires you to project manage and coordinate. It’s cheaper up front but more complex. Most people save money using a builder even though it feels more expensive.
What if my actual costs exceed budget?
You and your contractor should have a process for change orders. Anything outside the original scope requires written approval before work starts. If building control requests additional work, that’s usually the contractor’s responsibility if it’s a building code requirement, or your responsibility if it’s a change you made.
Can I reduce costs by providing my own materials?
Generally not. Contractors buy materials at trade prices, better than consumer prices. You’re paying for coordination and guaranteed quality. Providing your own materials often creates disputes about who’s responsible if materials are defective.
We work across London boroughs (West: Ealing, Harrow, Hillingdon, Hounslow; North: Barnet, Brent, and beyond). We’ll map out realistic costs for your property, break down where money actually goes, and identify where you have real flexibility.
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