Modular garden dwelling vs house extension in Ireland
Since 27 July 2026, both a rear house extension (Class 1, S.I. No. 338 of 2026) and a detached rear-garden dwelling (Class 3A, S.I. No. 340 of 2026) are planning-exempt up to 45 m². A detached modular unit (€65,000 to €105,000 for 35 m² turnkey) avoids knocking the back wall of your house and finishes on site in days rather than 4 to 6 months.
Both options now have a 45 m² planning exemption — here is how they compare on cost, time, disruption and resale.
Free, no obligation. One vetted modular supplier covering your county prices it, not five.
Configure your size, cost and planning exemption
Pick your floor area, specification and use. Shows our 13.5% VAT price guide and whether your build falls under Class 3 (up to 30 m² non-habitable) or Class 3A (32 to 45 m² habitable garden dwelling, S.I. No. 340 of 2026).
Guide price (incl. 13.5% VAT) & planning status
€84,000 to €112,000
Class 3A Exempt (35 m² within 32–45 m² band)
Based on 35 m² at our turnkey habitable unit guide rate of €2,400 to €3,200 a m² (including 13.5% VAT on supply and installation). Exempt from planning permission under Class 3A (S.I. No. 340 of 2026, in force 27 July 2026): a detached habitable dwelling of 32 to 45 m² in the rear garden. Total garden structures cannot exceed 45 m², the homeowner must live in the main house, it connects to the main house's services, cannot be sold separately or short-term let, requires 14 days prior written notification (with Eircode) to the council, must meet Building Regulations, and must be completed by 31 December 2030.
Send this for a written quote45 m² Class 3A modular dwelling vs 45 m² Class 1 rear extension
| Comparison point | Class 3A detached modular dwelling (32–45 m²) | Class 1 rear house extension (up to 45 m²) |
|---|---|---|
| 2026 planning limit | 32 to 45 m² detached in rear garden (S.I. 340 of 2026) | Up to 45 m² attached to rear of house (S.I. 338 of 2026) |
| Typical turnkey cost (35 to 40 m²) | €65,000 to €105,000 (incl. kitchen, shower room & base) | €75,000 to €135,000 (plus €12,000 to €25,000 if fitting a new kitchen) |
| Time on site in your garden | 1 to 3 weeks (built in factory over 4 to 8 weeks) | 14 to 24 weeks of digging, blockwork, roofing and plastering |
| Disruption to main house | Almost none — back wall stays closed; 1–2 days for service trench | High — knocking the back wall, steel RSJ beam, dust and scaffolding |
| Best for | Independent granny flat, adult child, carer or private work suite | Making the main house's kitchen-diner bigger |
Get a modular building price in 4 taps
Tap what you want to build, the size and your county. Your request goes to one vetted modular building supplier that covers your county, and they ring you with a written specification and price.
Questions people ask
Can I have both a rear extension and a Class 3A garden dwelling?
Each exemption has its own statutory conditions: Class 1 counts cumulative extensions to the house since 1 October 1964 (up to 45 m²), while Class 3A caps total detached garden structures at 45 m² and requires adequate remaining private open space. If your house already has an extension, ask your council for an €80 Section 5 declaration before building.
Get a written turnkey price for your modular building
One vetted supplier covering your county checks access, foundations and service connections, and gives you a written price.