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The true cost of building your house extension

25 October 2019

A well-planned extension can transform a home, but figuring out how it will impact your pocket is a tricky business. There’s plenty to consider when trying to unlock your property’s potential on a budget: planning permission, calling in the professionals, timing and more.

For example, something as simple as aiming for a spring/early summer build might save you money. You’ve a better chance of avoiding delays due to bad weather (though not always in Ireland!), which would push up labour costs as well as living costs if you’re renting elsewhere during construction.

Typical cost estimates don’t always tell the full story and these kinds of projects are notorious for taking longer than expected and arriving over-budget. Thankfully, with the right info and prep work, extending can be more than worth the effort. So, let’s take a look at those pesky hidden costs so you can save as much as possible…

Overly optimistic estimates

Arriving at an overall cost starts, quite literally, from the ground up. When you go to a builder for an estimate, many of them will simply look at the number of square metres of floor space to be extended and multiply it by a baseline figure. This is usually just over €1,000 per square metre for a single storey extension.

Instead, budget initially for around €2,000 per square metre for a new build. And that’s still excluding the purchase of fittings and finishes.

It’s worth remembering that builders are competing for the job at this point, and you’ll likely be getting a lowball number that doesn’t include any planning or design costs. Try to take everything into account before you get cracking.

With that in mind, we’ve partnered with to create the Home Improvement Calculator. It factors in the complexity of the job and quality of finish desired – giving you a completely tailored estimate. As for other watch-outs you may not have considered…

Professional help beyond your builder

If you’re looking for something other than a standard build, you may need the advice of a chartered architect. Expect to part with around €70 per hour for their expertise. Structural engineers command a similar fee, if required for a certain design.

Has your builder told you an architect is a waste of money? A hands-on builder taking care of the entire job might seem like a blessing for your budget, but they can be a false saving – getting a dedicated architect can help you avoid costly construction pitfalls.

Hiring a good project manager can set you back between €4,000-5,500. They’ll keep you on budget and on schedule, however, which might reduce the cost of your living expenses. Potentially one to consider on bigger jobs.

Planning permission

Rear extensions under 40 square metres do not generally require planning permission. For other jobs, you may need to hire an assigned certifier. Extending can also occasionally rub neighbours the wrong way. A build that affects someone’s sunlight or a boundary wall can lead to a dispute. Which then leads to extra fees. Be up-front and include them in the process to resolve issues early on.

Prepping your land

The area you’re extending into may need to be flattened before you can safely build. Excavating roughly 30 cubic metres of earth costs around €1,000. Building near trees, drains or other buried services can present costly obstacles. An architect will take this into account.

Boiler upgrades

Extra space means extra heating demands, which some energy-efficient boilers may not meet. It might also be necessary to replace boilers that are over 10 years old to ensure the whole house is effectively heated. Buying and installing a new boiler can cost up to €3,000. If your boiler is over 15 years old, you may be eligible for a grant under the Better Energy scheme.

Don’t forget a contingency fund

Having a contingency fund from the start is definitely recommended. Any hiccups during construction can mean additional builders’ fees, which you’re expected to cover. Putting aside 10-20% of your budget will give you the peace of mind that any hiccups won’t derail the project. So you can relax (somewhat!) and enjoy watching your vision take shape.


Lending criteria, terms and conditions apply. permanent tsb p.l.c. is regulated by the Central Bank of Ireland.

The content of this blog does not constitute advice and is for general information purposes only. Readers should always seek professional advice before relying on anything stated in the blog. Some of the links above bring you to external websites. Your use of an external website is subject to the terms of that site.

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