Construction costs on the Isle of Man are significantly higher than in most of the UK. The island's remote location, the cost of importing materials and specialist labour, and the relatively small size of the local construction market all contribute to higher per-square-metre build costs than you might find in, say, the North of England or the Midlands.

That said, it is possible to give realistic ballpark figures that help homeowners understand what they are likely to be dealing with before they commit to the design process. These are not quotes — they are informed estimates based on current market conditions and our experience of delivering projects across the Isle of Man.

For a straightforward single-storey rear extension with standard specification — concrete block construction, standard windows, basic kitchen or utility fit-out — you should budget a minimum of £3,000 to £3,500 per square metre of additional floor area. A 25 square metre extension would therefore cost a minimum of £75,000 to £87,500 in construction costs alone, before professional fees, planning fees, and contingency.

For a higher specification extension — contemporary design with structural glazing, bi-fold doors, underfloor heating, quality finishes, and a more complex structural arrangement — costs rise to £3,500 to £4,500 per square metre or more. The same 25 square metre extension might cost £87,500 to £112,000 at this specification level.

Two-storey extensions are generally more cost-efficient per square metre than single-storey, because the roof and foundations costs are spread over more floor area. However, they are more complex projects and typically require a longer programme.

Construction cost is only part of the picture. Professional fees — architect, structural engineer, and where relevant a quantity surveyor — typically add 12% to 18% to the construction cost. Planning fees are relatively modest, currently set by the Isle of Man Government at fixed rates, but they are an additional cost.

VAT at 20% applies to construction work in most cases, and this is a significant addition to the headline build cost. Some clients overlook this or assume it is included in contractor quotes — always clarify whether prices are quoted inclusive or exclusive of VAT.

A contingency of 10% to 15% of the construction cost is strongly recommended for any project. Even well-managed projects encounter unexpected conditions — particularly in older properties where what lies behind walls and under floors is not always predictable.

The single most effective way to control the cost of an extension project is to invest time in the design stage before going anywhere near a contractor. A well-designed, clearly specified project that goes to tender with complete information will attract more competitive prices than a vague brief that leaves contractors to guess at what is required — and then price in contingency to protect themselves.

At Modus Architects, we help clients establish a realistic budget at the very start of the design process, and we design to that budget rather than presenting beautiful drawings that turn out to be unaffordable. This is not always what clients want to hear at the beginning, but it produces far better outcomes than discovering a cost problem when tenders come back.