At Nestor Shanahan Auctioneers we believe a well-designed extension can transform a home. A bright kitchen-diner opening onto the garden, an extra bedroom or a ground-floor suite for an older relative can add real comfort and value. But an extension also brings extra questions, and the answers can affect your mortgage, your insurance and your ability to sell in the future. Before you make an offer on a property that has been extended, here are the five things we recommend every buyer checks.
1. Planning permission or a valid exemption
Every extension needs either planning permission or a valid exemption, and your solicitor will want to see proof of which applies. If permission was granted, ask for the grant and drawings, and make sure the extension as built matches what was approved, including any conditions attached.
Many smaller rear extensions are exempt from planning, but only within strict limits. From 27 July 2026, the exemption for extending a home increased from 40 to 45 square metres, subject to conditions and limitations. Those conditions cover matters such as height, distance from boundaries and leaving at least 25 square metres of rear garden, and previous extensions count towards the total. An extension built before July 2026 would have needed to meet the rules in force at the time it was built. If there is any doubt, an architect or engineer can assess the position, and the local council can issue a Section 5 declaration on whether works are exempt.
2. Building Regulations compliance
Planning and Building Regulations are two separate things. Even an exempt extension must comply with the Building Regulations covering structure, fire safety, insulation, ventilation and drainage. Ask the seller for a certificate or opinion of compliance from a suitably qualified architect, engineer or surveyor. Larger extensions are also subject to statutory building control procedures, including formal certification on completion. Missing paperwork is one of the most common causes of delays between sale agreed and closing, and your lender's solicitor will almost certainly ask for it.
3. Build quality and how it joins the original house
A good extension should feel like part of the home, not an afterthought. Pay close attention to where the new structure meets the old, as this is where problems most often appear. Look for cracks, gaps, damp patches or staining on ceilings and walls. Flat roofs are common on Irish extensions, so ask how old the roof covering is and whether there have been any leaks. Check that floors are level between the old and new sections, that doors close properly and that rooms feel comfortable rather than draughty. Your surveyor should inspect the extension carefully, including drainage and any signs of movement.
4. Heating and energy performance
An extension can significantly change how a home performs. Ask whether it was included when the current BER certificate was carried out, as an older certificate may not reflect the property as it stands today. Check how the extension is heated, whether radiators or underfloor heating are connected to the main system, and whether the space stays warm in winter. Large areas of glazing and rooflights bring wonderful light but can lose heat or overheat if poorly specified, so ask about the glazing type and insulation levels used.
5. Boundaries, neighbours and paperwork
Many extensions are built close to or right up to a boundary. Ask whether the neighbours agreed to the works, particularly if foundations, gutters or walls sit on or over the shared line, and check that the extension does not encroach on neighbouring land. Find out who built it and when, and request any guarantees or warranties for the roof, windows or structural work. Finally, remember that a larger home usually means a higher rebuild cost, so make sure your home insurance quote reflects the property as extended.
Tips for sellers
If your home has an extension, gather the paperwork before you go to market. Planning grants, exemption evidence, compliance certificates, an up-to-date BER and builder details all give buyers confidence and help avoid last-minute delays. If something is missing, speak to an architect or engineer now, as resolving gaps early is far less stressful than doing so under pressure once a buyer is waiting. Your auctioneer can also highlight the extension's best features, such as extra light, space and flexibility, in your listing.
The bottom line
An extension can make a home far more liveable and valuable, but only when it has been built properly and documented correctly. A few careful checks before you make an offer can protect your investment, keep your mortgage on track and ensure the extra space you are paying for will be an asset when you eventually sell.
If you would like to discuss buying or selling a property, contact us on 061 415337 or email info@nestorshanahan.ie or visit nestorshanahan.ie.
Disclaimer: This article is based on publicly available information and is intended for general guidance only. While every effort has been made to ensure accuracy at the time of publication, details may change and errors may occur. This content does not constitute financial, legal or professional advice. Readers should seek appropriate professional guidance before making decisions. Neither the publisher nor the authors accept liability for any loss arising from reliance on this material.
