What Will It Cost?

When planning an extension to your home, it is likely that you will have spent some time working out exactly what you want from that extra space. Before you go too far planning and building your extension, we would suggest that you have a rough idea of how much it is likely to cost.

Below is a rough guide to how much the building cost of an extension may be, based on the internal floor area on flat ground with no obvious problems:

• £1500 per sq m or £140 per sq ft – Basic structure completed by the builder, interior work finished by yourself

• £2100 per sq m or £196 per sq ft -Basic structure with simple internal, decorated finishes and electrics/heating completed by the builder

• £2350 per sq m or £219 per sq ft – More complex structure with intermediate level internal finishes and decoration completed by the builder

• £2900 per sq m or £270 per sq ft – More complex structure with luxury internal finishes and decoration completed by the builder

Both the pandemic and Brexit have had a significant impact on material and labour costs, making it harder to predict the cost of a build.

While the rough guide above provides an indication of costs, we highly recommend a getting budget estimate produced by a specialist for any agreed scheme. The budget estimate will provide a more accurate reflection of the build cost at that time, based on current pricing information, and is generally considered to be within 10% of the likely final cost. Although please note that there will be some caveats, allowances, and exclusions from it.

On top of the building cost, you will need to allow for all or some of the following fees:

• Architect’s fees – These will vary. 3HW’s fees for both Planning and Building Regulation applications start at £2500 plus VAT

• Planning permission – Check with your local authority. In Cornwall, this will be £258.00 (no VAT) from 6th December 2023

• Building regulations – Check with your local authority. In Cornwall this will range between £350 – £730 inc VAT depending on extension size

• Mining Report (required in Cornwall and in some other areas) – between £200- £300

• Other Specialists/Reports (if needed) – Your architect will advise you if you need any of these and their costs.

• VAT at 20% – most extensions will need VAT to be paid on top of the fees plus building costs

Your architect’s fee may seem like an unnecessarily large upfront cost, but if you have chosen a suitably qualified designer, they could save you from spiralling costs during the building of your extension and in later maintenance (see ‘Choosing Your Designer’).
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Please contact 3HW to get a more accurate budget estimate and to advise you on any extra costs your particular project may require. Our minimum fee of £2500 plus VAT will include highly detailed drawings and information, which will be sufficient for Planning and Building Regulations applications, as well as for the builder to construct from, thus reducing the likelihood of extra costs further into the project.