Planning an extension? What to consider before knocking down walls

Planning an extension?  What to consider before knocking down walls

Kya deLongchamps explores the essentials you should know before considering an extension to your home.

REAL, honest-to-goodness new space is habitable physical space. It’s not imagined, implied room conjured with luminous paint, tightly corseted storage or minimalist discipline. However, before you seize on exterior splinters of an acre and convert them to sleek interior roam, take time for a patient, considered approach to this nerve-mincing spend.


Near Zero Energy Building Regulations 2019: Planning permission aside, it’s vital to understand if your project is subject to the new demands of the NZEB 2019 statute, in place since last November. Most 40sm2 extensions will slide under the bar, but presume nothing — get professional advice.

If you are using a mortgage lender, they may ask about independent certification if your project comprises 25% of the existing footprint of the house and falls under the new regulation for the whole house to receive a deep retrofit to reach an overall B2 BER on completion. Even where you don’t have to comply? Every effort should be made to reach a good energy-efficient standard for the build to avoid future, expensive, remedial work.

Planning an extension?  What to consider before knocking down walls


With construction materials, salaries and fees on the rise, extending can be surprisingly expensive per square metre as groundwork proceeds and the original building is opened up to take the new elements. €1500 — €2,000 per new square metre (including VAT) for a standard 40m2 project in a single storey, would not be unusual even in top-notch timber or steel frame prefabricated builds.

Ensure you are fully aware and protect yourself from the unforeseen with a 5%-10% contingency fund. The final quote from any one-stop firm should be closely examined as it may well not include some second fix carpentry, plumbing and electrics, a kitchen, over-runs on provisional costings for materials, planning or other design fees.


Would you proceed with an extension if you had no guarantee of a return or even breaking even on your investment? There is a perceived ceiling on pricing of similar homes in most suburban areas. Over developing a house rather than finessing the existing building with a deep energy retrofit and new decor, can in some instances be a mistake. The effort to make your own life more convenient and pleasurable is the only thing has any real and certain value, especially in a small extension project. See the PSRA Property Price Register here:


Commission an architect. Few of us want to bolt a homogeneous pod onto the side of our home. If you are determined to get the best out of the extension and to potentially change the exterior styling of the house dramatically, let an architect handle the complex bridging of old and new.

There’s good reason an architect’s training is among the longest of all professions. A larger extension will quite likely shake up the layout of the entire floor it adjoins and may well demand new energy efficiencies in the rest of the house.

You will go through a brief which can result in more than one proposal. Payment is generally a percentage of the build cost, paid by lump sum or in staged payments if the architect is acting more comprehensively and coming on predetermined scheduled visits to certify the work;

Planning an extension?  What to consider before knocking down walls


Hire a quantity surveyor/project manager for a larger extension. QS Lisa O’Brian, formerly of RTEs Room to Improve, explained to me recently: “A QS keeps everyone in check with the finances — that’s the QS’s sole agenda, managing their client’s risk.

"The best compliment I get is when a client at the end of the project comments, ‘I don’t think we needed a QS — the job went really smoothly."

That’s when I pat myself on the back as I know what would have happened if they had not hired us.

We protect our clients from the (crucial) contractual humdrum.” As well as administering the contract, when presenting your project to a financial institution for a loan, your QS or a chartered building surveyor can also put together plans and quotes in a suitable format.


A builder needs as much detailed comprehensible information about the building and finishing of the house as is possible. In the hands of a competent architect you could be armed with detailed plans, a contract proposal, specifications, information on insurance, stage payments and even (best of all) a bill of quantities (itemising materials).

At least three firms should be tendered. The right contractor will have its trades on-site, on-time sequentially and will know everyone by the excellence of their workmanship. References and examples of past projects are vital even when you have a supervising professional’s useful network in play.

There’s a useful sample contract for a build project here (take it to your legal adviser,


Freeze the design: Work together when setting up a schedule of completion, obligations and staged payments with your builder in a full legal contract. Sticking to an agreed design is one of the most important things to keep things sweet on site. Blowing the budget and the designer’s mind with subtle changes means finances, planning stipulations and crucial relationships can smartly skid off the rails.

When tenders come back — brace yourself — you might not be able to afford the size or level of finish you want. If you want that 18th-century oculus-style window over a dog leg staircase, you will have to either find the extra money or undertake part of the work yourself. Going off standard in fixtures is expensive.


Converting a single storey single garage will garner you roughly 14 square metres (28sq m-30sq m in a double), enough for a generous first-floor bedroom, a multi-purpose playroom or study, or say a small dining area neatly abutting the kitchen. But, if you come to sell, the lack of indoor parking space and rough storage may grate with buyers.


Remember your garden is part of your home’s “breathing room” too.

An extension to the rear of 40square metres or less built without planning permission cannot reduce the open space at the back of the house to less than 25sq m.

If you have a pokey site, don’t expect to extend up to less than a metre from the garden fence or you will be left in a goldfish bowl with the walls closing in around you and a sternly worded planning authority notice dropping through your new back door.

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