The short version
- Attached rear extension up to 40m² — no permission needed (current).
- Detached garden building up to 25m², not for living in — no permission (current).
- Detached habitable structure of any size — full permission required (current). This is changing in 2026 but is not yet law.
- Tiny house on wheels for less than 6 months a year, no services connected — no permission. Lived in full-time = full permission.
- BER: any new dwelling needs A2 or better (nZEB).
What you can build without planning permission today
Ireland's exempt-development rules (Planning & Development Regulations 2001 as amended) carve out a handful of small-scale works that don't need a planning application. The ones that matter for modular & tiny home buyers are:
1. Attached rear extension up to 40m²
If you extend the back of your existing home by up to 40m² of floor area, you don't need permission, provided the extension is attached to the main dwelling, doesn't bring the rear private open space below 25m², and meets standard conditions on height, light, and overlooking neighbours. This is how a 40m² granny annexe attached to the kitchen wall works without a planning file.
2. Detached garden building up to 25m² — not for habitation
A standalone shed, garden office, gym, or storage building up to 25m² is exempt — but only if it isn't used as habitable accommodation. Sleeping in it counts as habitation. Once it has a kitchenette and a shower, councils will treat it as a dwelling.
3. Detached habitable structure (any size)
Right now, anything intended as living accommodation that isn't physically attached to the main house needs full planning permission. That includes a 30m² Baltic prefab dropped onto a back garden, a 38m² modular as a separate annexe, or a permanent log cabin used as a residence. Size doesn't get you out of the application — habitability does.
What's changing in 2026
Back-garden modular exemption: up to 40–45m²
The Department of Housing ran a public consultation on extending the exempt-development rules to detached habitable modular units up to 40–45m² behind an existing home. It closed in late 2025 with over 900 submissions, more than half supportive. Draft regulations were brought to Cabinet and discussed in the Dáil early 2026, and are at an advanced stage but have not yet been signed into law as of this writing (May 2026).
If and when these come in, the proposed conditions are likely to mirror the current rules: floor area below the cap (40 or 45m² depending on the final draft), located behind the main dwelling, retaining 25m² rear private open space, not blocking neighbours' light or overlooking them, and meeting building regulations including BER A2.
Watch this space — the Tánaiste and Housing Minister have publicly described the move as a way to "remove red tape" on modular housing. Until the SI is signed, plan as if the existing rules apply.
Tiny houses on wheels (THOW) and mobile homes
Temporary use: under 6 months / year, no services
A mobile home, caravan, or tiny house on wheels does NOT need planning permission if all of these apply: used for fewer than 6 months a year, not connected to public services (water, sewer, electricity), and EPA-compliant. Holiday units on private land sit in this bracket.
Permanent residence or fixed connection
Once a mobile home becomes someone's main residence, OR it's permanently connected to mains water/sewer/electricity, OR it's fixed to a foundation, councils treat it as a dwelling and full planning permission is required. Mobility doesn't matter — function does.
"Bed in a shed"
Don't try it. Local authorities actively investigate complaints about people living in unconverted detached structures, and unauthorised use of a 25m² garden building as a bedroom is one of the most common enforcement issues nationally. Once a structure is in habitation it loses its exempt status retrospectively, and the council can require its removal.
BER (Building Energy Rating)
Every new dwelling sold or rented in Ireland needs a BER certificate. New builds since November 2019 — including modular and prefab homes — must achieve a minimum BER of A2 under the nearly-Zero Energy Building (nZEB) standard. Most quality Baltic and Irish modular builds ship at A2 or A3 by default; the build spec sheet will tell you.
From 24 May 2026, the BER scale is being simplified. Subcategories disappear and the new top tier is A0, reserved for dwellings that use no fossil fuels (heat pump + electric only, typically). Modular builds with air-source heat pumps will often qualify.
Modular homes on agricultural land
Putting a dwelling on agricultural land is a change of use and almost always requires full planning permission. In rural areas, councils apply local-needs criteria from the County Development Plan — typically you need to demonstrate a connection to the area, a housing need, and the site has to meet road-sightline, septic-tank percolation, and well-water requirements. None of this is modular-specific; it applies whether you're putting up a kit-built timber-frame or a Baltic prefab.
What's the practical path?
For most ourhouse.ie buyers — granny flats, downsizing, garden offices that double as guest rooms — the practical answer for a typical 25-50m² modular today is:
- Attached annexe: under 40m², on the back of your existing house, no permission needed.
- Garden office or studio: under 25m², detached, not for sleeping in — no permission. (Yes you can have a heat pump and a small bathroom; no kitchenette + bedroom combo.)
- Standalone dwelling, granny annexe, holiday cabin: file a planning application. It's typically an 8-week decision plus a 4-week appeal window. Many providers in our directory will handle the documentation as part of their service.
- If you can wait 6-12 months: the proposed back-garden 40-45m² exemption may be law by late 2026, which would change the calculus significantly.
Sources & further reading
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