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A Detached House in the Rear Garden

A common question right now, and the answer changed on 27 July 2026. Here is the position today and the conditions that come with it.

Information, not advice. Much of the law described here is new and has not yet been tested in the courts or by An Coimisiún Pleanála, and it could be affected by challenge, amendment or further regulation. This page gives our reading of it. For a definitive answer on a particular property or application, engage with the relevant planning authority.
The rules changed on 27 July 2026. A detached house in the rear garden of 32–45 m² can now be exempted development, subject to conditions, including a duty to notify the planning authority at least 14 days before work starts. This is general information, not formal advice, take advice before ordering a unit.

The Position Today

Since 27 July 2026 a detached, self-contained dwelling in the back garden of a house can be exempted development. S.I. 340 of 2026 inserted a new Class 3A covering a detached house in the rear garden of a principal house, commonly referred to as a "modular" or "garden" home, of between 32 m² and 45 m², with no separate connection to utilities (condition 12). Separately, a garden room used as ancillary space (home office, gym, studio) now falls within a 30 m² limit, raised from 25 m², subject to height and open-space conditions. See the exemption checker for the full set.

In force  What changed on 27 July 2026 (S.I. 338 to 344 of 2026). The following can now be exempted development, each subject to the conditions of its class:
  • › Add a detached home of 32–45 m² in the back garden, with no separate utility connections and meeting the Building Regulations, including fire safety (Class 3A).
  • › Divide a house into a separate self-contained unit of at least 32 m².
  • › Extend a house by up to 45 m² (was 40 m²), and add a garden structure up to 30 m² (was 25 m²).

Two conditions do the real work, and both are easy to miss. The detached house in the rear garden and the sub-division each require the owner to notify the planning authority at least 14 days before work starts, on our reading that is a condition of the exemption, not a courtesy, and building first may mean the exemption never applied. The Class 3A exemption is also time-limited: the works must be started and completed by 31 December 2030 (conditions 1 and 2). It applies only where the main house is the owner's sole or main residence when work starts, and the detached house may be occupied only with the main house, may not be sold or subdivided separately and may not be used for short-term letting (conditions 3, 19 and 20).

Exempt from Planning Is Not Exempt from Everything

  • › Building & fire regulations apply. Exemption from planning is not exemption from Building Regulations, a habitable unit must meet them in full.
  • › Services and drainage. Water, wastewater and connections need to be dealt with properly.
  • › Planning conditions, protected structures and conservation areas can each remove the exemption: a condition on an earlier permission that limits exempted development (article 9(1)(a)(i)), works to a protected structure that would materially affect its character (section 57), or exterior works in an architectural conservation area that would materially affect its character (section 82). The full list is under Before Relying on Any Exemption.
  • › Renting it out or separating it in title can raise further planning and legal questions.
Our take. The exemption is available, but in our view it is not a free pass. The 14-day notification is the condition most likely to be overlooked, and a habitable 32–45 m² unit has to meet the Building Regulations in full, a materially higher bar than a garden room. Where a site is tight, overlooked, or in a conservation area, an application may be the safer route. A declaration from the planning authority under section 5 of the 2000 Act is the formal answer on whether particular works are exempt.
Sources: Citizens Information, altering a house · gov.ie, new exemptions in effect (27 July 2026) · S.I. 340 of 2026
Read on 3 October 2026 against the text of S.I. 338 to 340 of 2026.

A Public Information Service

These guides and tools are published as a public service and are kept to the primary legislation. They are not a free advice line, and individual queries are not answered by email or phone. For a general question about a particular property, the planning authority's planning department and Citizens Information are the starting points, and a declaration under section 5 is the formal answer. Where a matter needs professional work, the practice takes instructions on a fee basis: from developers, landowners, investors and public bodies, and from private clients, for example on retention or regularisation of unauthorised development or a change of use, and occasionally a one-off house, subject to capacity.

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