Thinking about extending your home but unsure how much it will cost? You’re not alone. Understanding how to estimate building costs for an extension can feel overwhelming, especially when you’re trying to plan realistically. At KT5 Construction, we work with homeowners across London and Surrey—many of whom are middle-class families juggling dreams and budgets. This blog breaks down the process clearly and simply so you can plan with confidence.

Key Factors That Influence Extension Costs

Size and Type of Extension

• • Rear, side, loft, or wraparound extensions each come with different price tags.

  • A single-storey extension typically costs between £1,800–£2,500 per square metre, depending on finishes and location.

Location

• • London-based builds can be up to 20–30% more expensive than elsewhere in the UK due to labour and material costs.

  • Tight access or listed building status also adds to pricing.

Specification and Materials

• • Standard finishes = lower cost.

  • High-spec or bespoke finishes = increased spend.

  • We always recommend clients consider where savings can be made without compromising durability.

How to Break Down Your Extension Budget

Pre-Construction Costs

• • Architectural plans: Save up to £3,000 with KT5—we offer free architectural or structural design when appointed as your builder.

  • Planning permissions (if required): ~£200–£500.

  • Structural calculations: £500–£1,500 depending on complexity.

Construction Costs

Break it into stages:

• • Groundworks & Foundations: ~20% of your budget.

  • Shell construction: ~40%.

  • Roof, windows, and doors: ~20%.

  • Interior finishes (electrics, plumbing, flooring, painting): ~20%.

Use this breakdown to spot where costs may creep and where KT5’s value engineering can save you money.

Common Budgeting Mistakes Homeowners Make

Underestimating VAT and Professional Fees

• • VAT at 20% applies unless your project is zero-rated (e.g., certain conversions).

  • Don’t forget surveyor, party wall, and inspection fees.

Forgetting Contingency

• • Always set aside 10–15% as a contingency.

  • Unforeseen ground issues or material price fluctuations can affect the final total.

How KT5 Construction Supports You

At KT5, we help clients understand building costs before a spade hits the ground.

1. Transparent quotes with clear itemisation
2. Free architectural or structural design worth £3,000
3. Expert cost-saving advice based on experience
4. Clear communication throughout

Our team ensures you don’t just build—you build smart.

Frequently Asked Questions

1. Can I build a 20m² extension for under £40,000?

Yes, depending on materials and spec. Talk to KT5 about low-cost, high-impact solutions.

2. How can I make cost savings without compromising quality?

Use value-engineered solutions like cost-effective insulation, modular construction methods, and standardised window/door sizes.

Final Words: Plan With Confidence

Knowing how to estimate building costs for an extension puts you in control. Whether you’re planning a kitchen extension in london or a rear build in Surrey, KT5 Construction offers guidance and solutions tailored to your budget.

We don’t just build—we partner with homeowners to create value from day one.

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Contact KT5 Construction today to book your free consultation and architectural design session.
Let’s bring your extension dream to life—on time, and on budget.