The true cost of your home extension
The 40-Square-Metre Myth: What Your Home Extension Will Actually Cost
If you are planning a home extension in Ireland, you have likely heard the "magic number" from architects: €3,000 per square metre. While this is a common starting point, it is often a "very loose figure" that fails to account for the complexities of a modern build. When you are looking at the standard 40-square-metre extension, the reality is that the final bill is more likely to land between €130,000 and €140,000.
Beyond the Basic "Square Metre" Price
The reason the standard estimates often fall short is that they don’t always calculate the high-spec features homeowners actually want. If you are envisioning a triple slider, for example, you aren't just paying for the glass; you are also paying for the additional steel required to support it.
Other factors that quickly drive up the cost include:
Glazing and Metal Roofing: These premium finishes are often not included in basic estimates.
Electrical Demands: Modern homeowners require significantly more sockets and complex lighting schemes than the "basic electrics" of the past.
Open-Plan Living: If your project involves making the downstairs of your existing property open-plan, the structural work involved adds another layer of cost.
The Hidden Cost of Regulations and Insulation
Pricing has shifted significantly since 2006 or 2007, when the standard was closer to €2,500 per square metre. Today, building regulations are much stricter. If you are building an A-rated extension onto an older home, you have to consider the thermal balance of the entire structure.
In many cases, it is advised to install external insulation on the existing house. There is little point in having a modern, warm extension if it is attached to a cold, uninsulated main building; you want to avoid having a "warm part and a cold part" of the home. Additionally, modern builds may require heat exchange units and high-level insulation that weren't standard a decade ago.
Navigating the Builder Market
Finding a reputable builder for a 40-square-metre job can be surprisingly difficult. Currently, some architects suggest that many builders are not interested in projects valued under €200,000 because there is so much other work available.
Because 40-square-metre extensions often fall under a planning exemption, they can attract a cohort of builders who prefer the "lack of oversight". To protect yourself, you must be diligent:
Check References: Even if an architect is not officially involved in the build, a reputable builder should be able to provide two or three references from architects or engineers.
Price Everything First: To avoid the "add-on and add-on" effect that frustrates homeowners, ensure every detail is discussed and priced before the build begins. This transparency is the only way to stop a disargeement once construction is underway.
What do you actually get for your €140,000?
Usually, this covers the walls, roof, glazing, electrics, and plumbing for a kitchen (though the kitchen units themselves are typically not included). Depending on the design, you might even manage to squeeze in a downstairs bathroom.
Planning an extension is like upgrading a vintage car with a modern, high-performance engine. If you don't ensure the rest of the car—the insulation, the structure, and the wiring—is prepared to handle that new power, you’ll end up with a very expensive machine that doesn't run smoothly.
Discussing the full scope of the work upfront ensures the "new" and the "old" work in perfect harmony.