Building an extension is one of the biggest investments you can make in your home. Get the budget right from the start and you will avoid the stress that comes with unexpected costs halfway through the project.

This guide covers realistic per-square-metre costs for different extension types in Ireland, along with the additional expenses that often catch people out.

Extension Costs Per Square Metre

As a general rule, budget between 2,000 and 3,500 euro per square metre for a standard single-storey rear extension in Ireland. This assumes a basic to mid-range specification including foundation, walls, roof, windows, electrics, plumbing, plastering, and basic finishes.

A typical 20 square metre kitchen extension therefore costs between 40,000 and 70,000 euro. A larger 40 square metre open-plan extension runs from 80,000 to 140,000 euro.

Two-storey extensions cost more overall but less per square metre, typically 1,800 to 3,000 euro per square metre because the foundation and roof costs are shared across two floors.

High-spec finishes such as large-format glazing, underfloor heating, or premium kitchen fit-out can push costs to 3,500 to 5,000 euro per square metre.

Additional Costs to Budget For

The build cost is only part of the total spend. You will also need to budget for architect or engineer fees (typically 8 to 12 percent of build cost), planning permission (65 euro application fee plus drawings), building control compliance, kitchen fit-out if applicable (10,000 to 40,000 euro), landscaping and making good the garden, and temporary accommodation if the disruption is significant.

Always add a contingency of 10 to 15 percent to your budget. Extensions almost always uncover unexpected issues once walls are opened up, particularly in older homes.

Planning Permission

Many single-storey rear extensions up to 40 square metres are exempt from planning permission under current exempted development regulations. However, exemptions come with conditions around height, distance from boundaries, and total site coverage. Always check with your local authority or an architect before assuming your project is exempt.

How Long Does an Extension Take?

A straightforward single-storey rear extension typically takes 10 to 16 weeks from breaking ground to handover. Two-storey extensions take 14 to 22 weeks. Add 8 to 12 weeks for planning permission if required, and 4 to 8 weeks for design and tender.

Getting Quotes

The best approach is to have an architect or engineer prepare drawings and specifications before going to tender. This ensures every builder is pricing the same scope of work, making comparison meaningful. Get at least 3 quotes and check references for similar-sized projects.