House extension cost: a price guide for London

Although there is no precise formula for obtaining an accurate cost for your extension (prior to obtaining a fixed quote for the project from a builder), the following figures give indicative levels for work in London: A creatively designed terraced house extension, featuring large areas of glazing may bear a construction cost of around £40,000.

The subsequent fit-out will be in addition to this construction sum. This figure will be more variable and depend on the preferences of the homeowner, for example, stone flooring will have a greater cost implication than timber alternatives. As a guideline, the fit-out cost may be between £10,000 and £20,000 for a small single storey extension.

A more accurate cost indication for your extension may be obtained at the planning stage of your project. At this time, the outline design is complete, and floor plans are ready to be submitted with a planning application to the local authority. A builder will then be able to give an indicative estimate for the work.

It is important to note that any builder's estimates for the complete build cost that you obtain at this stage can only be an indicative. Builder's estimates can be revised until you have agreed a written quote for the work.

An estimate may develop into a fixed quote for your extension when detailed, post-planning architectural drawings have been produced and a builder has visited the property to discuss your requirements and survey the site conditions. This quote will form part of a contract between the client and the builder that will be signed by both parties before work begins on your extension.

From this stage, many clients choose a builder independantly, and are prepared to oversee the construction process themselves. Alternatively, your architect may produce a formal tender package to invite a number of experienced builders to submit quotes for the project. Your architect will also be able to provide a full site supervision service until the project's completion if desired.

Once work has begun on site, payments are normally made to the builder at monthly intervals.

It is wise to ensure that there is a contingency sum included in any quote obtained with a builder. This sum will only be used to pay for unforseen additional costs should they occur. On average, around 5% of the quote is sufficient for a typical extension. If the contingency was not required, it will be deducted from the final bill once works are complete.

Please visit 'Our ideas' to see Architecture for London's previous house extension plans, or contact us to book an initial design and planning consultation at your home today.

HOUSE / KITCHEN EXTENSIONS BUILDING REGULATIONS PLANNING
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