If you’ve ever tried to work out the cost of a home extension or renovation project, you’ll know how confusing and opaque it can feel. “How much will it cost?” often gets a vague “Well… it depends.”
So let’s break the fog.
This post is your no-nonsense breakdown of typical costs, based on projects in Southwest London — including building work, professional fees, consent charges, and everything in between.
👉 Book a free discovery call to get early clarity and support on your next steps.
1. Building Costs
This is the price of the construction work itself — what your builder will quote for labour, materials, and subcontractors.
| Project Type | Typical Build Cost (per m², ex VAT) |
|---|---|
| Rear Extension | £1,800 – £3,500 |
| Loft Conversion | £1,500 – £2,500 |
| Interior Refurbishment | £500 – £1,600 |
|
These ranges depend on the complexity of installation, number of units, layout changes, plumbing, lighting, finishes, and appliances. High-end projects or bespoke cabinetry can push the upper limits — while budget solutions (like flat-pack ranges) can keep things lean.
To estimate your overall build cost:
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Multiply your project area by the rates above
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Add in any kitchen, bathroom or joinery items
Then multiply by 1.6 to allow for VAT, design fees and contingency.
2. Design Fees (Architect + Consultants)
These are approximate fees for the professionals helping you design and manage your project.
| Service | Typical Cost (ex VAT) | VAT | Total (Inc VAT) |
|---|---|---|---|
| Architect – Planning Design & Application | £2,000 | £400 | £2,400 |
| Architect – Thames Water Drawings | £500 | £100 | £600 |
| Architect – Building Regs Drawings | £1,200 | £240 | £1,440 |
| Architect – Tender Pack for Builders | £1,800–£3,000 | £360–£600 | £2,160–£3,600 |
| Interior Design (Optional) | £1,000 | £200 | £1,200 |
| Architect - Site Monitoring (per10 hrs.) | £1,000 | £200 | £1,200 |
| Structural Engineer (Steel calcs, drawings) | £2,000 | £400 | £2,400 |
| Measured Survey | £600 – £1,500 | £120 – £300 | £720 – £1,800 |
Tip: fees can scale with complexity — your architect can advise on scope and what's necessary.
3. Consent Fees (Statutory Applications)
Some of these are fixed by law or external bodies — but all are essential to getting your project off the ground (legally!).
| Consent | Budget Cost (ex VAT) | VAT | Total (Inc VAT) |
|---|---|---|---|
| Planning Application (Householder) | £206 | £0 | £206 |
| Lawful Development Certificate (optional) | £103 | £0 | £103 |
| Building Control Fee (based on £150k build) | £900 | £180 | £1,080 |
| Thames Water Build Over Agreement | £286 | £57.20 | £343.20 |
| CCTV Drainage Survey & Report | £330 | £66 | £396 |
| Party Wall Surveyor (per neighbour) | £1,200 | £240 | £1,440 |
| CDM Principal Designer Role | £1,000 | £200 | £1,200 |
4. Other Project Costs
| Item | Cost | ||
|---|---|---|---|
| Freeholder Consent (if developing a flat) | £TBA | ||
| Insurance for Construction Works | £300 – £600 | - Usually provided by specialist brokers | |
| Solicitor's Fees (if required) | £TBA |
Ballpark Example
Here’s a rough total for a 40m² single-storey rear extension, mid-spec:
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Build Cost: £2,500/m² × 40m² = £100,000 + VAT = £120,000
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Design + Professional Fees = £12,000–£16,000
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Consents + Surveys = £4,000–£6,000
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Contingency (10%) = £12,000
VAT (20%)
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TOTAL = £177,600 – £184,800 (give or take)
Why Transparency Matters
One of the biggest myths in residential projects is that all these costs go to “the architect.” In reality, your architect helps you orchestrate these moving parts — but many of the fees go to other specialists, regulatory bodies, and third-party consultants.
That’s why we break everything down clearly for our clients — so they can make informed decisions, plan ahead, and avoid costly surprises.
Ready to Explore Your Ideas?
If you’re thinking about a home extension or renovation and want to understand what it could cost — let’s chat.
👉 Book a free discovery call to get early clarity and support on your next steps.
