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Can you put a 40sqm modular home or log cabin in your back garden?

Possibly. Ireland's new Class 3A exemption covers qualifying new detached auxiliary dwellings from 32–45m², but it is not a blanket permission for every 40sqm cabin. Existing sheds, open space, ownership, boundaries, access, services and intended use all matter.

In force since 27 July 2026. Created by S.I. No. 340 of 2026.

Primary sources reviewed: 4 September 2026 · Republic of Ireland · General information, not legal advice

The short version

The phrase “40sqm planning law” is common search language, but the signed rule is broader and more precise. It creates a temporary Class 3A exemption for constructing, erecting or placing one detached auxiliary house in the rear garden of a principal house. The qualifying dwelling itself must be at least 32m², while the combined cap and other conditions can limit the workable maximum to less than 45m². Development must be commenced and completed in the statutory relevant period, which ends on 31 December 2030.

The important change: a 40sqm modular home, tiny home or log cabin can sit inside the permitted size band. It qualifies only if it is a permanent detached auxiliary dwelling and every Class 3A condition is met. The product name and floor area do not decide the planning position by themselves.
Do not mix up three different figures: 32–45m² is the Class 3A auxiliary-dwelling band, 45m² is also the current ceiling for certain attached rear extensions, and 30m² is the separate ceiling for qualifying non-habitable Class 3 garden structures. Each route has its own conditions.

The conditions that drive the checker

32–45m² rangeThe new detached dwelling must meet the minimum, while the combined cap can reduce the available maximum.
Existing Class 3 structures countSheds, garages, stores and similar structures previously placed under Class 3 count with the dwelling toward 45m².
25m² must remainThe development cannot reduce private open space reserved for the occupants below 25m².
Owner's main residenceThe principal house must be the property owner's sole or main residence when development starts.
Rear garden and permanentThe unit must be detached in the rear garden and cannot be temporary, such as a caravan or mobile home.
No prior house subdivisionThe Class 3A exemption cannot be used where the principal house has been subdivided.
Height and boundariesMaximum height is 4m for a tiled/slated pitched roof, otherwise 3m. The structure and boundary-facing windows require at least 0.6m clearance.
Access and servicesIndependent access must remain within the property, no new road access may be created, and services must link to the principal house.
Occupation restrictionsThe dwelling must be occupied with the main house, cannot be sold or subdivided separately, and cannot be used for short-term letting.
Notify before workThe planning authority must receive the prescribed notification at least 14 days before works commence.

How an existing shed changes the maximum

The signed text does not simply give every garden a separate 45m² dwelling allowance. Paragraph 7 combines the new Class 3A dwelling with structures previously constructed, erected or placed under Class 3.

Estimated maximum = lower of (45 − existing Class 3 area) and (current usable open space − 25)
15m² Class 3 shed: 45 − 15 = 30m². Because 30m² is below the 32m² Class 3A minimum, the supplied arrangement has no qualifying Class 3A range.
10m² Class 3 shed: 45 − 10 = 35m². The potential numerical range is 32–35m², provided the open-space and every other condition also pass.
Do not guess the shed's status. A structure built under Class 3, a structure covered by planning permission, and an unauthorised structure do not have interchangeable planning histories. If you cannot document which applies, seek a site-specific review.

Existing cabins and conversions are different

Class 3A describes construction, erection or placing of a detached house commenced and completed in the relevant period. It does not say that an existing cabin becomes authorised retrospectively, and it does not describe changing a non-habitable shed into a dwelling.

If the structure already exists or its use is changing, check its planning history before relying on any exemption. A Section 5 declaration is the formal route for asking a planning authority whether specified development is or is not exempted development.

Planning exempt does not mean document-free

Class 3A expressly preserves Building Regulations, fire-safety requirements and Building Control Regulations. Fingal County Council's implementation guidance says an auxiliary habitable dwelling requires a Commencement Notice with Additional Documentation, including the nominated design certifier, assigned certifier, competent builder and a Certificate of Compliance on Completion.

For an off-site manufactured dwelling assembled on site, Fingal's guidance also identifies an Agrément certificate or compliance with an Irish Standard. Confirm the exact documentation for the chosen system before paying a deposit.

Separate checks remain: planning exemption, Building Control, technical compliance, wastewater capacity, utilities, insurance, title restrictions and supplier documentation are separate questions.

A safe order of work

  1. Measure the current private open space, all existing Class 3 structures, the proposed footprint, height and boundary distances.
  2. Retrieve the principal house's planning permissions and confirm whether any condition or Article 9 restriction removes exempted-development rights.
  3. Have the design, access, drainage, services and Building Control route checked by competent professionals.
  4. If the planning position is uncertain, request a Section 5 declaration. Councils currently list a fee; confirm the current form and charge with your own authority.
  5. Submit the prescribed Class 3A notification at least 14 days before starting, then keep the notification, design and completion records together.

What if you plan to rent the garden dwelling?

Class 3A planning status and Rent-a-Room tax treatment are separate. Revenue's current guidance says a self-contained unit adjacent to, but not attached to, the main residence cannot qualify for Rent-a-Room Relief. A detached Class 3A dwelling therefore does not become eligible for the €14,000 relief merely because it shares the garden, Eircode or services with the main house.

Do not treat €14,000 as an automatic tax-free allowance. Read our source-led guide to the attachment test, the gross-receipts threshold, family lets, normal rental tax and RTB registration: Garden homes and Rent-a-Room Relief in Ireland.

Frequently asked questions

Can I put a 40sqm modular home in my back garden without planning permission?

Possibly, but not because it is 40sqm or modular. A 40sqm home is inside the 32–45m² size band. Every other Class 3A condition on the property, owner, existing structures, open space, design, access, services, use and notification must also be met.

Do the new rules cover log cabins and tiny homes?

Class 3A does not depend on the marketing label. A new modular home, log cabin or tiny home may qualify only if it is a permanent detached auxiliary dwelling and every planning condition is satisfied. A caravan or mobile home is not included.

Is a 45m² dwelling automatically exempt?

No. The 45m² figure is a cap inside a class with numerous cumulative conditions. Existing Class 3 structures and the remaining-open-space requirement can reduce the workable maximum.

Does any garden shed count?

The wording refers to other structures previously constructed, erected or placed under Class 3. If the shed's planning basis is unknown, the checker should return a review flag rather than assume it counts—or assume it does not.

Can I remove a shed and regain the area?

Removing a shed may change the physical and numerical position, but demolition, planning history, site geometry and the final proposal still need checking. Treat this as a potentially resolvable route, not an automatic approval.

Can I use the dwelling for Airbnb or another short stay?

No under Class 3A: paragraph 19 states that the detached dwelling may not be used for short-term letting.

Can it be rented long-term?

The planning instrument expressly prohibits short-term letting and requires occupation in conjunction with the main house. Long-term tenancy, tax and RTB treatment are separate issues. Revenue's current guidance excludes a self-contained unit adjacent to but not attached to the residence from Rent-a-Room Relief. Read the detailed tax guide and obtain advice on your arrangement.

Does the 14-day notice confirm that I qualify?

No. Notification is a condition of the exemption, not a planning decision or certificate that every condition has been met.

Primary sources

This page and checker provide general educational information based on sources reviewed on 4 September 2026. They do not determine that development is exempt, replace professional advice, or remove the need to comply with Building Regulations, Building Control, fire safety, utilities, wastewater, title, insurance or other legal requirements.