While not legally mandatory for all extensions, an architect or architectural designer is highly recommended. They bring expertise in design, planning regulations, building control, and project management. They can help maximise your space, ensure structural integrity, and often save you money in the long run by avoiding costly mistakes and optimising the design.

Extension costs vary widely based on size, complexity, location, and materials. As a very rough guide, a single-storey extension can cost anywhere from £1,500 to £2,500+ per square metre. This doesn't usually include kitchens, bathrooms, or high-end finishes. Always get detailed quotes from builders and factor in all professional fees and a contingency fund.

A standard planning application in England typically takes 8 weeks to process. Larger or more complex applications can take up to 13 weeks or even longer. This does not include the time taken to prepare plans or make any revisions requested by the planning department. Always factor this lead time into your project schedule.

While possible for very small, simple projects if you have significant building experience, it's generally not recommended for a full home extension. Extensions involve complex structural work, adherence to building regulations, and coordination of various trades. Mistakes can be costly and even dangerous. Using experienced professionals ensures the work is safe, compliant, and durable.

If your proposed extension will be built over or within 3 metres of a public sewer, you will likely need a 'build over agreement' from your local water authority. This ensures that their access to the sewer for maintenance is protected and that your building work won't damage the sewer. It's a legal requirement, and you should apply for it well in advance of construction.