Homeowner guide

Garden room, garden office or extension: which needs planning permission?

Updated 2026-07-18 · checked against the sources below

A detached garden room (often called a 'Shomera' after the well-known brand) and an attached extension are treated completely differently by Irish planning law. The garden room is usually the easier route - but with a hard catch around living in it, and a major change arriving on 27 July 2026 when the exempt size rises to 30 m² and a brand-new exemption for habitable garden dwellings begins. Here is how to choose.

Garden rooms today (until 26 July 2026): the 25 m² rule

Class 3 of the 2001 Regulations exempts garden structures - sheds, home offices, gyms, playrooms - when all conditions are met:

Maximum 25 m², cumulative across every such structure in the garden.
Behind the front wall of the house, and at least 25 m² of private open space must remain.
Height: 4 m with a tiled or slated pitched roof, 3 m with any other roof.
The use must be incidental to the enjoyment of the house - and the structure shall not be used for human habitation. No sleeping accommodation, no granny flat, no letting.

Can I work from a garden office? Yes - with one boundary

A garden room used as a personal home office, gym or playroom sits comfortably within Class 3: Fingal County Council's guidance says exactly that, and the Government's own description of Class 3 lists 'home office'. The boundary is business use that changes the character of the property - employees working there, customers calling, commercial signage. That can amount to a material change of use needing permission, separate from the structure itself.

What changes on 27 July 2026

Two things. First, the Class 3 limit rises from 25 m² to 30 m² (S.I. 338 of 2026) - conditions otherwise unchanged, including no habitation.

Second, and genuinely new: Class 3A (S.I. 340 of 2026) exempts a detached HABITABLE garden dwelling for the first time - subject to strict conditions:

Floor area at least 32 m², and at most 45 m² combined with any other garden structures.
Occupied only in conjunction with the main house: it cannot be sold or let separately, short-term letting is barred, and the owner must live in the principal house.
Same 4 m / 3 m height limits; at least 0.6 m from any boundary; no new road entrance; no separate utility connections; independent level access.
It must not be temporary in nature - a caravan or mobile home does not qualify.
Building regulations, including fire safety, fully apply.
A 14-day prior notification to the planning authority is required before starting - a brand-new procedural step - and the scheme runs to 31 December 2030.

Does prefabrication matter?

For planning purposes, essentially no. The exempting classes cover the 'construction, erection or placing' of a structure, so a prefabricated unit craned in and placed is judged on the same size, height, siting and use conditions as a block-built one. The one construction-method rule is Class 3A's: a habitable garden dwelling must not be temporary in nature, so a mobile home or caravan cannot ride the new exemption.

Building regulations are a separate question. A detached garden building of 25 m² or less used for recreation or storage is exempt from the building regulations; a garden office arguably sits outside that wording, and a habitable Class 3A dwelling is fully inside the building-control net. For a specific case, a Section 5 declaration from your council (EUR 80, answered in 4 weeks) is the definitive answer.

So: garden room or extension?

The honest decision frame:

Office, gym, studio or playroom - a garden room is usually the fastest route: exempt to 25 m² today, 30 m² from 27 July, no application, no commencement notice.
Extra living space that is part of daily life - kitchen, bathroom, bedroom - an attached extension (exempt to 40 m² today, 45 m² from 27 July, within its conditions) keeps everything under one roof and one set of rules.
Independent accommodation for a family member - from 27 July, the Class 3A garden dwelling is the first planning-free route, if you can live with its conditions; otherwise it is a planning application.
Beyond any of the limits, or in a protected structure or conservation area - it is an application either way, and the extension guide's process applies.

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Useful tools

Construction cost calculator

Compare the rough cost of the extension route vs the garden room.

Solar Site Checker

A garden-room roof can carry panels - check the site's solar potential.

Home Energy & Grant Report

The full SEAI grant picture for the main house.

Quick answers

Do I need planning permission for a Shomera-style garden office?

Usually not: a detached garden room used as a personal home office is exempt up to 25 m² today (30 m² from 27 July 2026) if the height, siting and open-space conditions are met. Running a business with staff or customers from it can need permission as a change of use.

Can someone sleep in a garden room?

Not under Class 3 - habitation is expressly barred, whatever the marketing says. From 27 July 2026 the new Class 3A allows a genuinely habitable garden dwelling of 32 to 45 m², but only with its strict conditions - including a 14-day notification to the council, no separate sale or letting, and full building regulations with fire safety.

Does a prefabricated or modular building have different planning rules?

No - planning judges size, height, siting and use, not construction method. The exception is that a temporary structure such as a caravan or mobile home cannot use the new habitable garden dwelling exemption.

Do garden rooms need a commencement notice?

A planning-exempt garden building that needs no fire safety certificate does not need a commencement notice. A habitable Class 3A dwelling is subject to building control including fire safety - expect formal guidance on its exact paperwork as the new class beds in, and ask your building control authority before starting.

Sources

House extension rules 2026 How to apply for planning permission Section 5 declaration (glossary)Exempted development (glossary)

General guidance, not legal or planning advice. The regulations and your local authority's interpretation bind; conditions and local rules (protected structures, architectural conservation areas, flood zones) can change the answer for a specific property. When in doubt, ask your council for a Section 5 declaration or talk to a planning consultant.