Modular Buildings glossary of terms & specifications
15 technical terms, material grades and Irish standards explained in plain English so you know exactly what is on a written quote.
Sorted A to Z for quick reference when comparing specifications.
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Key modular buildings terms explained
13.5% Construction Supply-and-Install VAT
The reduced Irish VAT rate that applies when a modular builder both manufactures and installs your garden room or modular dwelling on site (compared with 23% VAT on supply-only flat-pack kits).
14-Day Prior Council Notification (Class 3A)
The mandatory written declaration (including your Eircode) submitted to your local county council at least 14 days before starting a 32 to 45 m² Class 3A habitable garden dwelling.
BCAR (Building Control Amendment Regulations)
Irish Building Control sign-off by an Assigned Certifier required for standalone modular homes on their own site and extensions over 40 m².
Cedar & Thermowood Rainscreen Cladding
Kiln-dried Western Red Cedar, Siberian larch or thermally modified Nordic pine fitted over a ventilated cavity and breather membrane for a 30+ year timber exterior.
Class 3 Exempt Garden Room (Up to 30 m²)
The non-habitable rear-garden exemption under S.I. No. 338 of 2026 (raised from 25 m² to 30 m² on 27 July 2026) for home offices, gyms and studios not used for sleeping.
Class 3A Habitable Garden Dwelling (32 to 45 m²)
The detached rear-garden habitable dwelling exemption under S.I. No. 340 of 2026 (in force 27 July 2026 until 31 December 2030), allowing a 32 to 45 m² single-storey home where the owner occupies the main house.
EPDM Single-Ply Rubber Roof Membrane
A seamless vulcanised synthetic rubber sheet bonded across flat and low-pitch modular roofs with a 40 to 50 year waterproof lifespan.
Ground Screw Foundation System
Galvanised helical steel piles driven 1.2 m to 2.5 m into the ground in a single day using a hydraulic torque head — eliminating concrete excavation and curing time.
Light-Gauge Galvanised Steel Frame (LGSF)
Cold-rolled structural steel studs and roof joists engineered to millimetre accuracy in a factory for a rot-proof, non-combustible 60+ year structural skeleton.
Macerator / Pumped Foul Waste Pipe
A small-bore pressurised waste pump used when a rear-garden Class 3A bathroom sits lower than the main house's gravity foul sewer invert.
Part L NZEB Thermal Envelope
Wall, roof and floor insulation plus airtightness membranes and triple or A-rated double glazing engineered to keep heat loss low year-round.
Private Open Space Rule (25 m² Minimum)
The mandatory condition under both Class 3 and Class 3A that at least 25 m² of clear private garden space must remain behind the main house after the modular unit is installed.
Section 5 Exempted Development Declaration (€80)
A formal written ruling from your county council (€80 fee, decided within 4 weeks) confirming in writing that your garden room or Class 3A dwelling is exempt from planning permission.
SIPs (Structural Insulated Panels)
High-strength factory sandwich panels with a rigid polyurethane or polyisocyanurate insulation core bonded between two structural OSB boards.
Volumetric Pod vs Panelised Cassette Delivery
Delivering a complete factory-finished building by hiab crane over the house (volumetric) versus carrying insulated wall, floor and roof cassettes through a narrow side passage (panelised).
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Questions people ask
Can I build a 45 m² modular home or granny flat in my back garden without planning permission?
Yes, since 27 July 2026, if you meet the Class 3A conditions in S.I. No. 340 of 2026. A detached habitable garden dwelling between 32 m² and 45 m² in the rear garden of a house is exempt from planning permission provided total garden structures do not exceed 45 m², the owner lives in the main house, it connects to the main house's services, it is not sold separately or used for short-term letting, you give the local council 14 days prior written notification with your Eircode, it meets Building Regulations, and work finishes by 31 December 2030.
What is the difference between Class 3 (30 m²) and Class 3A (32 to 45 m²)?
Class 3 (updated by S.I. No. 338 of 2026) covers non-habitable garden rooms, home offices, gyms and sheds up to 30 m² — you cannot sleep or live in a Class 3 room. Class 3A (S.I. No. 340 of 2026) covers a detached habitable garden dwelling (granny flat) of 32 m² to 45 m² with a kitchen, bathroom and bedroom.
How much does a modular building or garden dwelling cost in Ireland?
An insulated, fitted garden office or room (12 to 30 m²) costs €18,000 to €55,000 including 13.5% VAT. A turnkey habitable 32 to 45 m² Class 3A modular dwelling with kitchen, shower room, foundations and service connections costs €65,000 to €135,000 (€2,400 to €3,200 a m²).
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