House Extension Cost Calculator
Use our quick extension calculator and get a realistic budget range fast.
Estimated budget range
Extension Cost Calculator
Planning more space at home starts with one big question: what is it likely to cost? Our extension cost calculator gives homeowners a simple way to build an early budget before committing to plans, drawings, or contractor conversations.
Use it to explore the likely scale of your project, compare different ideas, and understand which choices may affect your extension budget. It is designed to give you a practical starting point, then help you move towards a more accurate home extension quote when you are ready.
Get a Clearer Estimate Before You Start Planning
An extension can change how your home works every day, but the final price depends on more than floor area. A kitchen extension, extra bedroom, utility space, or open-plan living area can all involve different levels of structural work, services, finishes, and approval checks.
The extension cost calculator keeps the first step straightforward. Enter the key details you know now, review your extension cost estimate, then refine the project as your plans become clearer.
What the Calculator Takes Into Account
Your estimate may be shaped by:
- the approximate size of the extension
- whether the design is single-storey or multi-storey
- the type of room or rooms being added
- structural changes, openings, and roof details
- kitchens, bathrooms, heating, electrics, and drainage
- windows, doors, insulation, and energy performance
- access, ground conditions, and existing property layout
- the quality of fixtures, finishes, and materials
- planning, building control, or party wall considerations
From Rough Budget to Confident Next Step
Start with a quick calculation. If the result fits your plans, you can then look more closely at layout, specification, and timing. If the estimate is higher than expected, the calculator can help you adjust the size, finish, or scope before you spend money on detailed design work. When you are ready, use your extension budget as a basis for a more detailed conversation. A tailored quote can then reflect your property, your preferred specification, and any local requirements that apply to the work.What Can Change Your Final Price?
An online extension calculator is a guide, not a fixed quote. Your final price can change after surveys, drawings, structural checks, or discussions with building professionals. Some extensions may be possible under permitted development rules, while others may need planning permission. Most extension projects also need building regulations approval, and some properties may require party wall notices or additional local checks. These steps can affect both budget and timescale, so it is best to allow for them early.Frequently Asked Questions
Is the extension cost calculator a fixed quote?
No. It gives an early estimate based on the information entered. A fixed quote normally needs more detail about the property, design, specification, access, and site conditions.
What details do I need to use it?
You can begin with basic information, such as the type of extension, approximate size, number of storeys, and intended use. More detail will usually make the extension cost estimate more useful.
Will I need permission for my extension?
It depends on the property, the design, previous alterations, and local rules. Some projects may fall within permitted development, but others require formal approval. Always check before work begins.
Can it help me compare different options?
Yes. Try different sizes, rooms, and specifications to see how house extension costs may change. This makes it easier to balance wish-list features with a realistic budget.