The expense of constructing a side or rear house extension in London is subject to considerable variation, contingent on various factors such as the dimensions and intricacy of the extension, the materials utilized, and the location of the property. Typically, the cost of constructing an extension in London is higher compared to other regions of the UK due to elevated construction expenses and property values.

To give a ballpark figure, building a rear house extension in London can cost between £1,500 to £2,600 per square meter for an elementary design and standard quality materials. However, the cost can rise to £4,000 or more per square meter for a luxurious design and top-quality materials.

Other factors that influence the cost of constructing an extension in London consist of:

• Planning permission and building regulations charges
• Structural engineer fees
• Groundwork and foundation expenses
• Roofing and cladding fees
• Windows and doors expenses
• Plumbing and electrical work costs
• Finishing and decoration charges

It's worth noting that these are simply approximate estimates, and the actual cost of building an extension in London is dependent on the specific details of your project. It's always recommended to obtain a comprehensive quote from a reputable company with a lengthy track record before commencing any construction.