Auxiliary habitable dwelling, the legal definition explained.
Since 27 July 2026, a detached auxiliary dwelling in your back garden can be exempt from planning permission under Class 3A of S.I. No. 340 of 2026. Knowing the definition, and every condition attached to it, is the difference between a back-garden home that is lawful exempt development and one that becomes a planning enforcement problem.
This is the legal-definition reference. For the full picture, costs, the rental return, models and the buyer's guide, start with our complete guide to back-garden modular homes in Ireland.
Auxiliary Habitable Dwelling
A detached self-contained residential unit, between 32 and 45 square metres in floor area, located in the rear garden of an existing principal house, with services (water supply, electricity, drainage) linked to the services of the principal house, exempt from planning permission under Class 3A, provided every prescribed condition is met.
Source: Class 3A, Planning and Development (Exempted Development (Act of 2000)) (No. 3) Regulations 2026, S.I. No. 340 of 2026. In operation from 27 July 2026; the exemption runs to 31 December 2030.
The conditions that must all be met
Exemption from planning permission applies only where every condition below is satisfied. Failing any one of them means the unit is not exempt and a planning application is required. This is a plain-English summary; the statutory text in S.I. No. 340 of 2026 governs.
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The unit is detached from the principal house
The auxiliary dwelling must be a freestanding structure. An attached extension to the principal house is a different category (covered by the 45 m² principal extension exemption).
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The floor area is between 32 and 45 square metres
The minimum floor area of 32 m² ensures the unit is a habitable dwelling rather than a glorified shed. The maximum of 45 m² is the policy cap on the exemption, units above 45 m² require full planning permission.
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The unit is located in the rear garden of the principal house
The exemption does not apply to side-garden or front-garden placement. The "rear garden" location is fundamental to the planning rationale (privacy, overlooking, streetscape impact).
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Services are linked to those of the principal house
Water supply, electricity and drainage must connect through the principal house's supplies, not as independent connections to the public mains. This service linkage is what legally distinguishes an auxiliary dwelling from an independent second dwelling on the plot.
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The unit is a habitable dwelling
The exemption applies to residential use only. A commercial use (workshop, retail, office let to a third party) is not exempt and would require its own planning consent.
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The principal house is your home
The main house must be the owner's sole or main residence when work begins. A landlord cannot use Class 3A to add a unit behind a rented-out house.
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The height limits are respected
No more than 4 metres with a pitched tiled or slated roof, or 3 metres for any other roof. A flat-roofed expandable unit at 2.48 metres sits well inside the limit.
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The council is notified 14 days before work starts
Notifying the planning authority at least 14 days before development begins is mandatory, and most councils now publish a Class 3A notification form. This is separate from the Commencement Notice required under building control.
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Occupancy stays tied to the main house
The unit may only be occupied with the principal house. It cannot be sold or subdivided separately, and it cannot be used for short-term letting.
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Works finish by 31 December 2030
The exemption is time-limited. Works must be commenced and completed within the exemption period, which currently ends on 31 December 2030.
Important. Even where the headline conditions are met, site-specific factors, distance from boundaries, height, overlooking of neighbouring properties, established trees, Protected Structure status of the principal house, may still trigger a need to consult your local authority. Every Ériu Modular Homes project starts with a site eligibility review against the Class 3A conditions.
Five financial things to know, before you sign anything
1. Rent is taxable income
Revenue's Rent-a-Room guidance states that a self-contained unit not attached to the property cannot qualify for the relief. Plan on rent from a detached garden home being taxed as ordinary rental income, with allowable expenses deducted. What that is worth as a yield depends on your all-in installed cost, so we model it per quote; confirm the tax position with your accountant.
2. Outside the RTB's remit
The RTB has stated that renting a qualifying Class 3A unit does not come under its remit and does not need RTB registration. Short-term letting is prohibited, so this applies to long-term lets. It is a major operational difference from a conventional residential rental.
3. Separate LPT charge
Revenue has confirmed a permanent garden home suitable for use as a separate dwelling attracts its own LPT charge and property ID, payable even when unoccupied. Annual LPT is based on the unit's open-market value and the local authority's adjustment factor.
4. Insurance must be updated
Home insurance on the principal house must be updated to declare the new structure, failing to do so can invalidate cover on the principal house. Most Irish insurers will quote an uplifted premium rather than require a separate policy.
5. Mortgage and resale implications
Most main Irish mortgage lenders will not currently extend a mortgage on a property containing a second self-contained dwelling on the same title. Many homeowners use chattel mortgages, personal loans or credit union finance to fund the auxiliary unit specifically to avoid affecting the principal house mortgage.
Always confirm with your accountant and solicitor. Tax treatment and conveyancing implications depend on individual circumstances. We provide a written LPT, insurance and mortgage briefing pack with every quote, but it is not a substitute for professional advice on your specific position.
Two legal categories. Two different rule sets.
It is essential to understand whether you are building an auxiliary habitable dwelling or a garden room/garden office, because the legal treatment of the two is fundamentally different.
| Auxiliary Habitable Dwelling | Garden Room / Office | |
|---|---|---|
| Floor area exemption | 32–45 m² | Up to 30 m² |
| Intended use | Self-contained habitable dwelling | Non-habitable: office, gym, studio, hobby room |
| Kitchen / bathroom plumbing | Yes (linked to principal house) | Optional, but not for self-contained residential use |
| Building regs (TGD Parts A–M) | Full compliance required | Partial, applicable parts only |
| BER certificate | Required | Not generally required (non-habitable) |
| Separate LPT band | Yes, own LPT charge (Revenue) | No |
| Rent-a-Room eligibility | No, Revenue excludes units not attached to the house | No (not a habitable residential unit) |
| Insurance impact | Material, must be declared | Should still be declared |
Auxiliary dwelling, questions answered
Last reviewed: September 2026
What is an auxiliary habitable dwelling in Irish planning law?
An auxiliary habitable dwelling is a detached, self-contained home of between 32 and 45 square metres in the rear garden of an existing principal house, with services (water, electricity, drainage) linked to the principal house. Since 27 July 2026 it can be exempt from planning permission under Class 3A of S.I. No. 340 of 2026, provided every condition is met, including notifying the planning authority at least 14 days before work starts. The unit must be occupied with the main house and cannot be sold separately or used for short-term letting.
What is the difference between an auxiliary dwelling, a granny flat and a garden room?
Auxiliary habitable dwelling is the precise legal term in the 2026 regulations, it is the planning-law category. Granny flat is the colloquial term for the same physical structure, typically used when the intended occupant is a relative. Garden room or garden office is a non-habitable structure under a separate legal category (the 30 m² garden structure exemption), it lacks the residential plumbing and kitchen specification of a habitable dwelling. The legal category determines what planning rules apply, what building regulations apply, whether LPT applies separately, and how the unit is treated for insurance and mortgage purposes.
What are the qualifying conditions for an auxiliary dwelling exemption in Ireland?
Under Class 3A of S.I. No. 340 of 2026, the conditions include: the unit is detached and in the rear garden of the principal house; floor area is between 32 and 45 square metres; the principal house is the owner's sole or main residence when work begins; services are linked to those of the principal house; height does not exceed 4 metres with a pitched tiled or slated roof, or 3 metres otherwise; enough private garden space is kept; the planning authority is notified at least 14 days before work starts; the unit is occupied only with the main house, is not sold or subdivided separately and is not used for short-term letting; and works are commenced and completed by 31 December 2030. Protected Structure status and other site factors can still take a project outside the exemption.
How is rental income from an auxiliary dwelling taxed in Ireland?
Plan on it being taxable. Revenue's Rent-a-Room guidance states that a self-contained unit not attached to the property cannot qualify for the relief, so rent from a detached garden home is treated as ordinary rental income, with allowable expenses deducted. The letting does sit outside the RTB's remit: the RTB has stated that a qualifying Class 3A unit does not need to be registered. Short-term letting is not permitted. Confirm your position with a qualified accountant before relying on a projected return.
Does an auxiliary dwelling get its own Local Property Tax (LPT) band?
Yes. Revenue has confirmed that a permanent garden home suitable for use as a separate dwelling attracts its own LPT charge. It is valued in its own right, gets its own property ID, and LPT is payable even when it is unoccupied. The amount depends on the unit's open-market value and the local authority's LPT adjustment factor.
Can an auxiliary dwelling be built using a modular or container-based system?
Yes. The exemption applies to the dwelling regardless of construction method, provided the unit meets the Irish Building Regulations (TGD Parts A to M) and the conditions of the exemption itself. Expandable steel-frame container homes and traditional modular construction are both eligible. The exempt-development status relates to the legal classification of the dwelling, not to the construction method used to build it. Imported MMC units must still carry CE marking, a Declaration of Performance and a written TGD compliance pack to be lawfully placed on an Irish residential site.
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