Extension Cost Calculator – Estimate Your Home Extension Costs in London

Thinking about extending your home but not sure where to start with the budget? Our free Extension Cost Calculator helps you estimate build costs for your project in minutes. Whether you’re planning a rear extension, loft conversion, or even a two-storey addition, our calculator provides an initial guide to help you plan effectively before speaking to builders or architects.

Why Use Our Home Extension Cost Calculator?

Extension projects in London vary significantly in cost depending on design, size, and specification. Our tool gives you a quick and transparent starting point by considering:

• New floor area created (ground floor extensions are typically higher in cost than upper levels)
• Size of structural openings – from simple door openings to large glazed features
• Internal alterations – such as wall removals and layout changes
• Additional works like kitchens, bathrooms, plumbing, and electrics
• Choice of finishes – from basic UPVC doors to slim frame glazing

By adjusting these options, you’ll receive a realistic cost range tailored to your project, helping you understand feasibility from the outset.

How Accurate Is the Extension Calculator?

While no online tool can replace a detailed quotation, our extension calculator is designed using real build cost data from projects across London and the surrounding boroughs. We’ve included a range from lower-cost builds to high-end finishes so you can see how your budget might shift. Once you have an idea of costs, the next step is to discuss your plans with professionals. At Detailed Planning, we can prepare designs, secure planning permission, and produce building regulation drawings to take your project forward with confidence.

Factors That Influence Your Extension Costs

Beyond size and specification, several key factors will affect the price of your extension:

• Location: Costs in London tend to be higher due to labour and material demand.
• Planning Permission: Some extensions fall under permitted development rights, while others require a full planning application.
• Structural Complexity: Removing walls, underpinning foundations, or integrating a garage conversion can all impact cost.
• Specialist Features: Rooflights, slim-frame glazing, and bespoke joinery add design value but increase budgets.

Planning Your Extension Budget

Our calculator is an excellent starting point, but every project is unique. For a tailored breakdown, we recommend a feasibility study and design consultation. We can help you:

• Explore different design options to suit your budget
• Submit planning applications to your local London borough
• Produce accurate drawings for tendering to builders
• Guide you through construction with professional advice

If you’re considering adding value to your home, explore our guides to kitchen extensions, garage conversions, or full house renovations.

Get Started With Your Extension Estimate

Try our free Extension Cost Calculator today to see how much your dream project could cost. Once you have a figure in mind, book a consultation with Detailed Planning to explore design solutions, planning strategy, and technical drawings that make the most of your investment.

Disclaimer: Figures from the calculator are intended as a guide only. Actual construction costs will depend on your site, design, and chosen contractor.