# Structural Engineer for Extension Dublin | Foundations & BCAR > Structural engineer for extensions in Dublin: foundation and steel design, the opening into the house, and BCAR sign-off over 40 m². Fixed fee before the visit. URL: https://structuralengineer.ie/structural-engineer-for-extensions/ # Structural Engineer for Extensions in Dublin Foundations designed for the ground you actually have, steel for the opening into the house, and a Chartered Engineer who can sign the BCAR certificates. [Call 01 270 9764](tel:+35312709764) [Get a fixed fee](#quote) _Image: Foundation excavation in boulder clay for a rear extension behind a Dublin semi-detached house, mini digger on the lawn_ - I.S. EN 1990 Basis of design - I.S. EN 1991 Actions (incl. snow) - I.S. EN 1992-1-1 Concrete design - I.S. EN 1993-1-1 Steel design - I.S. EN 1995-1-1 Timber design - I.S. EN 1997-1 Geotechnical design - TGD A Structure - BCAR S.I. 9 of 2014 Last updated: 14 October 2026 · Reviewed by Joachim Louw, CEng MIEI [StructuralEngineer.ie](https://structuralengineer.ie/) is the structural engineer for extensions across Dublin, Kildare, Meath and Wicklow. Joachim Louw (CEng MIEI) designs the foundations, the steel for the opening into the existing house, and the roof and floor structure. On extensions over 40 m² he can act as designer and assigned certifier under the Building Control Regulations. Since 27 July 2026, rear extensions up to 45 m² no longer need planning permission, but the structure still has to meet the Building Regulations in full. Published 2026 Irish cost guides put extension structural design at €1,200–€3,500. Single-storey rear extensions start at about €1,200, and two-storey or wraparound designs sit at the top of the range. Your fee is fixed in writing before the visit. ## What happens on the first visit On the first visit the engineer looks at three things: the ground where the extension will sit, the rear wall that will be opened up, and what that wall carries from the floors and roof above. Bring the architect's or designer's drawings if you have them. If not, the visit starts from your sketch and the measurements taken on site. - Check the existing rear wall and what the first-floor joists, roof and upstairs walls put onto it - Agree the width of the opening into the house and where the new steel will bear - Look for drains, manholes and trees near the footprint, since all three change the foundation design - Check the boundary, including whether the new wall sits on or beside a party wall - Note how the existing house is built: cavity block, solid brick or timber frame - Decide whether trial holes are needed to confirm the ground before the foundations are designed ## Foundations: designed for the ground, not a default depth Most Dublin extensions sit on glacial boulder clay, which is a good founding material when it is firm and undisturbed. Strip or trench-fill foundations designed to I.S. EN 1997-1 are the usual answer, often founded around a metre below ground on clear sites. Three things change that. Trees on clay soils draw moisture out in dry summers and the ground shrinks, so foundations near trees go deeper, depending on the species, height, distance and how plastic the clay is. Drains running under or beside the footprint need the foundation taken below them or bridged over them. Made ground, old backfill and soft river deposits may need a raft or piles instead of strip footings. Where new meets old, the existing house has had decades to settle and the extension has not. The junction is detailed so the two can move slightly without cracking: a tie-in or a movement joint, chosen to suit the foundations on each side. ## The opening into the existing house The rear wall of a semi or terrace usually carries the first-floor joists and part of the roof. Opening it up for a kitchen extension means a steel beam, often a goalpost frame of beam and posts, carrying those loads down to new or existing foundations. The steel is sized to I.S. EN 1993-1-1 for strength and deflection, with padstones or base plates sized for bearing, and a propping sequence for the builder while the wall comes out. Fire protection is specified to Technical Guidance Document B. ## Flat roofs, rooflights and roof lanterns Flat-roof joists are usually C24 timber sized to I.S. EN 1995-1-1 for span, deflection and the snow load for the site under I.S. EN 1991-1-3. Large rooflights and lanterns break the joist run, so the opening needs trimmers or a steel frame to carry the cut joists. Wide spans may call for glulam beams or steel. All of it goes on the drawing, so the builder isn't sizing timber on site. ## Planning, BCAR and certification Rear extensions up to 45 m² are exempted development from 27 July 2026, subject to conditions. The limit counts every extension built since 1 October 1964, at least 25 m² of private open space must remain, and some estates have planning conditions that remove exempt rights. Side and front extensions still need permission. Extensions over 40 m² come under the Building Control (Amendment) Regulations. That means a Commencement Notice with compliance documentation lodged 14–28 days before work starts, a designer and an assigned certifier, an inspection plan, and a Certificate of Compliance on Completion. Alternatively, the owner can file an opt-out declaration. A Chartered Engineer can take both certifying roles. Between 40 m² and 45 m², the new planning exemption and the 40 m² BCAR threshold overlap. Confirm with your building control authority how they treat your extension before you start. For smaller exempt extensions there are no statutory certificates, so the engineer's opinion of compliance is what your solicitor will ask for when you sell. Opening the back wall for big glazed doors? See [openings for sliding doors](https://structuralengineer.ie/rear-wall-opening-for-sliding-doors/). For the rules, read the [45 m² extension exemption](https://structuralengineer.ie/guide/45m2-extension-exemption/) and [BCAR for extensions over 40 m²](https://structuralengineer.ie/guide/bcar-extension-over-40m2/). ## What's included - Site visit by a Chartered Engineer - Foundation design, with trial-hole advice where needed - Steel design for the opening: beams, posts and goalposts - Roof and floor structure design - Structural drawings and calcs your builder can price from - Steel schedule for the fabricator - Inspection of foundations and steel at key stages - Opinion of compliance, or BCAR certificates over 40 m² Typical market range: **€1,200–€3,500** (extension structural design; single storey from about €1,200). Source: BuildTech; Banrach cost guides, 2026. Fixed fee confirmed in writing before the visit. ## Builder's design, online calcs or an engineer on site How the structure of an extension gets decided, and what each route leaves you with when you sell or remortgage. | Check | Builder designs as they go | Online calcs, no visit | Chartered Engineer on site | | --- | --- | --- | --- | | Ground, trees and drains checked before foundation design | No | No | Yes | | Opening into the house designed from actual loads | No | assumed | Yes | | Foundation excavations inspected before concrete | No | No | Yes | | Steel inspected before it is covered | No | No | Yes | | Can act as BCAR designer and assigned certifier | No | No | Yes (CEng) | | Certificate a buyer's solicitor will accept | No | limited | Yes | Sources - [BuildTech: residential structural engineer cost Ireland](https://buildtech.ie/blog/residential-structural-engineer-cost-ireland) - [Banrach: structural engineer cost in Ireland](https://banrach.ie/how-much-does-a-structural-engineer-cost-in-ireland/) - [Kelleher Associates: 2026 planning exemption regulations](https://www.kelleherassociates.ie/new-2026-planning-exemptions-regulations-come-into-effect/) - [gov.ie: Commencement Notices](https://www.gov.ie/en/service/ebb3b-commencement-notices) - [S.I. No. 365 of 2015 (opt-out for domestic extensions)](https://www.irishstatutebook.ie/eli/2015/si/365/made/en/print) Talk to the engineer 01 270 9764 Mon–Fri 8:00–18:00. Callback the same working day. [Call now](tel:+35312709764) Fixed fee confirmed in writing before the visit. Standards we design to - I.S. EN 1990 Basis of design - I.S. EN 1991 Actions (incl. snow) - I.S. EN 1992-1-1 Concrete design - I.S. EN 1993-1-1 Steel design - I.S. EN 1995-1-1 Timber design - I.S. EN 1997-1 Geotechnical design - TGD A Structure - BCAR S.I. 9 of 2014 Before you book: costs and rules - [Extension Engineer Fees in Ireland: What a Structural Engineer Charges (2026)](https://structuralengineer.ie/guide/extension-structural-engineer-cost/) - [45m2 Extension Exemption: What Changed in July 2026](https://structuralengineer.ie/guide/45m2-extension-exemption/) - [BCAR Extension Rules: Assigned Certifier or Opt-Out Over 40m2?](https://structuralengineer.ie/guide/bcar-extension-over-40m2/) - [Party Wall Works in Ireland: Putting a Steel Beam into a Shared Wall](https://structuralengineer.ie/guide/party-wall-steel-beam-ireland/) - [How Much Does a Structural Engineer Cost in Dublin? (2026)](https://structuralengineer.ie/guide/structural-engineer-cost-dublin/) How the job runs ## How an extension is designed and signed off - 01 ### Call and fixed quote Send drawings or a sketch. You get a fixed engineering fee in writing. - 02 ### Site visit Ground, drains, trees, the rear wall and the boundary are checked on site. - 03 ### Design pack Foundation, steel and roof design with calcs, drawings and a steel schedule. - 04 ### Inspections and sign-off Foundations and steel are inspected before they are covered, then the opinion of compliance or BCAR certificates are issued. What the work involves ## Typical stages on site Illustrative images of typical work, not photographs of specific client projects. _Image: Steel goalpost frame in the opening for a rear extension on an Irish semi_ Steel goalpost frame in the opening for a rear extension on an Irish semi _Image: Concrete poured into trench foundations for a Dublin rear extension_ Concrete poured into trench foundations for a Dublin rear extension _Image: Flat-roof joists with a trimmed opening for a roof lantern on a new extension_ Flat-roof joists with a trimmed opening for a roof lantern on a new extension _Image: Finished kitchen extension with roof lantern and sliding doors in a Dublin house_ Finished kitchen extension with roof lantern and sliding doors in a Dublin house ### Typical before and after _Image: Before: plain rear elevation of a 1980s semi-detached house in Dublin_ Before Illustrative _Image: After: the same house with a single-storey rendered rear extension, roof lantern and sliding doors_ After Illustrative Planning an extension? Get the foundations and steel designed before builders price it. Fixed fee confirmed in writing before the visit. [Call 01 270 9764](tel:+35312709764) [Request a fixed fee](#quote) Questions ## Extension structural engineer: common questions **Do I need a structural engineer for a single-storey extension?** In practice, yes. The foundations, the opening into the house and the roof all have to comply with Building Regulations Part A, and most builders will not price an extension without structural drawings. Over 40 m², BCAR requires a designer and assigned certifier unless you opt out. When you sell, the buyer's solicitor will ask for an engineer's opinion of compliance. **How deep do extension foundations need to be?** That depends on the ground, not on a standard figure. On firm boulder clay with no trees nearby, strip or trench-fill foundations are often around a metre deep. Near trees on clay, over made ground, or beside drains, they go deeper or change type. The engineer sets the depth after seeing the ground, and inspects the excavation before concrete is poured. **Can I build a 45 m² extension without planning permission?** Yes, at the rear, from 27 July 2026, subject to conditions. The 45 m² includes any extension built since 1 October 1964, at least 25 m² of private open space must remain, and height and boundary limits apply. Some estates have planning conditions that remove exempt rights, so check the original grant. The Building Regulations apply either way. **What is an assigned certifier and do I need one?** For extensions over 40 m², the Building Control (Amendment) Regulations require an assigned certifier: a registered architect, registered building surveyor or Chartered Engineer. The certifier inspects to an agreed plan and signs the Certificate of Compliance on Completion. The owner can opt out by declaration, but the Building Regulations still apply and certification at sale gets harder. **What areas do you cover?** Dublin and the Greater Dublin Area (Kildare, Meath and Wicklow). Site visits are booked from the Blackrock office. Site visit usually within 5 working days. ## Related services [### Load-Bearing Wall Removal Steel beam and padstone design, propping, inspection and a signed certificate for knock-throughs.](https://structuralengineer.ie/load-bearing-wall-removal/) [### Openings for Sliding Doors Widening a rear wall for sliding or bifold doors: goalpost frames and lintels.](https://structuralengineer.ie/rear-wall-opening-for-sliding-doors/) [### Opinions of Compliance Engineer's certificate for past works when you sell, remortgage or regularise an extension.](https://structuralengineer.ie/opinion-of-compliance/) Next step Get a fixed fee for your extension Call [01 270 9764](tel:+35312709764), or send the details and we'll call you back. Request a fixed engineering fee Prefer to talk? [Call 01 270 9764](tel:+35312709764). A live person answers during office hours.