House extension glossary of terms and specifications
15 technical terms, planning rules and Irish building standards explained in plain English so you know what is on a written quote.
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- 15extension & building terms defined
- 45 m²Class 1 exempted rear limit
- 0.18Part L wall & floor U-value
- 13.5%reduced VAT rate
Key house extension terms explained A to Z
25 m² Private Open Space Requirement
Even when an extension stays within 45 m² in floor area, you must leave at least 25 m² of unbuilt private open garden space to the rear of the house for the extension to remain exempt from planning permission.
45 m² Cumulative Extension Floor Area Rule
Since 27 July 2026, the 45 m² exempted development allowance is cumulative across the life of the house since 1 October 1964. Any previous rear room, sunroom or converted attached garage built after the original house was finished is deducted from your remaining 45 m² allowance.
Architect / Engineer Stage Payment Valuation
Irish residential extension contracts are paid in 4 to 6 certified stages (foundations and floor slab, wall plate and roof structure, weathered shell with windows fitted, first-fix and plastering, practical completion, and 2.5% to 5% retention held for 6 to 12 months).
BCAR Commencement Notice Threshold (45 m²)
A Commencement Notice must be lodged on the Building Control Management System (BCMS) between 14 and 28 days before starting any extension that requires planning permission or exceeds 45 m². Exempt extensions of 45 m² or less do not require a Commencement Notice.
Builders' Finish vs Turnkey Specification
A builders' finish quote covers foundations, walls, roof, insulation, external windows and doors, first-fix plumbing and electrics, plaster skimming and concrete floor screed. A turnkey quote adds fitted kitchen units, sanitaryware, tiling, finished flooring, internal doors and painting.
Foul and Surface Water Drain Diversion / Build-Over
On most Irish semi-detached and terraced houses, the main foul sewer and storm drain run across the back patio just outside the original rear wall. Building an extension over them requires either diverting the pipework around the new foundations or bridging the pipe with concrete lintels and relocating the inspection chamber outside.
Opinion on Compliance with Planning and Building Regulations
The standard Law Society certificate signed by the supervising architect or chartered engineer upon completion, required by banks and solicitors whenever you remortgage or sell the extended house.
Party Wall / Boundary Eaves & Foundation Detail
When building right up to a shared side boundary, the extension gutter, fascia and foundation toe must not overhang into your neighbour's property without written consent; builders use a box parapet gutter and eccentric strip footing kept inside the boundary line.
RSJ (Rolled Steel Joist) & Padstone Goalpost Frame
When knocking through the original back wall of a house to create an open-plan room, a structural engineer sizes Universal Beams or a bolted steel goalpost frame on high-strength concrete padstones to carry the upper masonry wall and floor joists.
Schedule 2 Part 1 Class 1 Exempted Development
The statutory exemption in the Planning and Development Regulations allowing a rear house extension of up to 45 m² (increased from 40 m² on 27 July 2026) without planning permission, provided all conditions on previous extensions, garden space, height and boundary windows are met.
Section 5 Declaration of Exempted Development
A formal written ruling from your County or City Council (fee €80, statutory decision period 4 weeks) confirming that your proposed extension drawings qualify as exempted development. Useful for future house sale conveyancing when building near the 45 m² limit.
TGD Part L Elemental U-Value Method (0.18 / 0.16 / 1.4)
Where an extension is under 25% of the existing house floor area, compliance with Building Regulations Part L is achieved by meeting the elemental backstop U-values: 0.18 W/m²K for walls, 0.16 W/m²K for roofs, 0.18 W/m²K for floors and 1.4 W/m²K for windows and rooflights.
Two-Storey Rear Boundary Rule (2 m Distance)
In a two-storey exempted rear extension, the upper floor cannot exceed 12 m² on a semi-detached or terraced house (20 m² on a detached house) within the 45 m² total, any upper-floor window facing a boundary must be at least 7 m from that boundary, and the two-storey structure must sit at least 2 m inside any party boundary.
Two-Thirds VAT Rule (13.5% Building Service vs 23% Appliances)
The main extension build, structural steel, windows and permanently fitted cupboards qualify for the 13.5% reduced VAT rate under Revenue's two-thirds rule. Freestanding or plug-in appliances (ovens, hobs, fridges, dishwashers, washing machines) and professional fees are taxed at 23% VAT.
Warm-Deck EPDM or Fibreglass (GRP) Flat Roof
Modern flat extension roofs place 120 mm to 150 mm of rigid PIR insulation above the structural timber deck and vapour control layer, topped with a single-ply EPDM rubber membrane or joint-free GRP fibreglass laminate with a 20 to 30 year lifespan.
Plan your house extension
Four quick taps, and your plan fills in as you answer: what it costs, whether it needs planning permission and how long it takes. Contact details come last, and only if you want a builder to price it.
25 m² single-storey rear, mid-range finish: €52,000 to €65,000, including VAT and fees.
How the estimate is worked out
Every kind of extension has a build rate per m² for each region of Ireland, VAT at 13.5% included. A standard finish uses the lower half of that range and mid-range the upper half; high spec or a wall of glass moves the build to the glazed rate. Design, engineer and BER fees go on top, with anything you tick (an en-suite, a kitchen, a steel beam into the house) priced at your finish. A garage conversion gets one price for the whole job, since the walls and roof are already standing. Totals are rounded to the nearest €1,000. Use it to plan with. It isn't a quote: a builder prices your house after seeing it.
Questions about extension specifications
Does a garage conversion count towards my 45 m² exempted development allowance?
Under Schedule 2, Part 1, Class 1 of the Planning and Development Regulations (from 27 July 2026), converting an attached domestic garage to the side or rear of a house for use as part of the house is included in the 45 m² cumulative floor area allowance. If you convert a 15 m² attached garage and also build a rear extension without planning permission, the combined floor area of both cannot exceed 45 m².
How close to my neighbour's boundary wall can I build a single-storey or two-storey extension?
A single-storey rear extension can be built right up to the side boundary line provided no part of the foundation, wall, fascia or rainwater gutter overhangs the boundary into your neighbour's side (builders use an internal box parapet gutter along the boundary wall), and ground-floor windows facing a boundary sit at least 1 metre back. For an exempted two-storey rear extension, the upper storey must be set back at least 2 metres from any party boundary.
What is the difference between a builders' finish quote and a turnkey extension quote?
A builders' finish quote covers everything up to a plastered, watertight shell with screed floors, glazed doors and windows, radiators and white sockets, leaving you to buy and fit your own kitchen, floor tiles and paint. A turnkey quote includes the finished flooring, tiling, sanitaryware and decoration.
What happens if the existing foul sewer or storm drain runs under the footprint of my new extension?
Almost every Irish estate house has a 100 mm or 150 mm foul sewer and storm pipe running across the back garden 1 to 2 metres behind the original back wall. Your builder will open the existing inspection chamber at survey stage: any manhole must be moved outside the new extension footprint, and pipes passing through foundation trenches are bridged with concrete lintels and flexible rocker pipes.
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