Since the end of July, homeowners across Cork have had a real, workable route to building a granny flat in the back garden without applying for planning permission. For families in Douglas, Ballincollig, or Carrigaline weighing up space for an elderly parent or an adult child struggling to get on the property ladder, that’s a meaningful change. It’s no surprise the new granny flat regulations have drawn so much interest.
Being exempt from planning is only one part of the picture, though. Building regulations, building control, and fire safety requirements all still apply. They’re also the parts people most often overlook once the planning question feels settled. Here’s what still needs working through before anything is built.
A quick recap of what Class 3A allows
The Class 3A exemption, introduced under S.I. No. 340 of 2026, came into force on 27 July 2026. It covers one detached auxiliary dwelling of between 32 and 45 square metres in the rear garden of a house that is the owner’s main residence. At least 25 square metres of private garden has to remain. Any existing sheds or garden structures built under the older Class 3 rules count towards the 45 square metre cap.
The unit must be occupied in conjunction with the main house. It can’t be sold or subdivided separately, and it can’t be used for short-term letting. The work also has to be started and finished before 31 December 2030. We covered the planning side in more detail in our guide to Ireland’s new planning exemptions.
Planning exempt does not mean building regs exempt
The Class 3A regulations keep building regulations, fire safety requirements, and building control regulations fully in place. The Department of Housing has published separate guidance on how the building regulations apply to auxiliary dwellings, and fire safety is one of the main issues it highlights.
In practical terms, a 45 square metre granny flat is a new dwelling. It’s held to the same standards for structure, fire safety, ventilation, drainage, and energy performance as any other home. Before work starts, a Commencement Notice has to be lodged with the local building control authority, and Cork County Council’s building control team handles these for the county. Local authority guidance published since the rules changed indicates that a habitable auxiliary dwelling needs a design certifier, an assigned certifier, and a certificate of compliance on completion.
This is the most common misunderstanding since the rules changed. “Exempt” is heard as “no paperwork”, when really one set of paperwork has been removed and the rest still stands.
Three scenarios, three different answers
Fire safety requirements depend less on the size of the space and more on how it’s used, and whether it forms part of one home or two.
Renting out a spare room in your own home
If you’re letting a bedroom in the house you live in, the property remains a single dwelling. A building used as a dwelling, other than a flat, is exempt from needing a fire safety certificate, so no certificate is needed here. Working smoke alarms and a clear escape route still matter for anyone living under your roof.
A granny flat used by a family member
This is where people tend to assume the least is involved. Say you’re building a 40 square metre unit at the end of the garden in Blackrock for your mother. Because it’s a detached dwelling rather than a flat, it may not need a formal fire safety certificate. It still has to be designed and built to meet the fire safety part of the building regulations, including safe means of escape, fire detection, and adequate separation from the main house and site boundaries. It also goes through building control like any other new home. No certificate doesn’t mean no requirements, which is why this scenario is less clear-cut than it first appears.
Converting a house into a duplex
If you’re splitting an existing house into two units, one above the other, the building is no longer treated as a single private dwelling. Each unit becomes a flat, and the exemption for dwellings no longer applies. In most cases a fire safety certificate will be required before work begins, and a Disability Access Certificate usually follows where a fire safety certificate is needed. Applications in the county go through Cork County Council’s building control applications process, and it makes sense to lodge both together. This applies even where planning permission isn’t needed.
What to check before you start
Get advice on where your specific plan sits before committing to a design or signing with a builder or modular supplier. The same 40 square metres can fall into quite different categories depending on how it will be used. It’s also worth confirming the project qualifies for Class 3A in the first place. An existing shed, a small garden, or a condition on the original planning permission can all rule out the exemption, and in that case a full planning application is the route forward.
Don’t assume “exempt” means no professional input is needed. Someone still has to certify the design and the finished work, and it’s far easier to get these details right on paper than to fix them once the unit is built.
If the unit is for a parent with limited mobility, think about access from the outset. A granny flat won’t need a Disability Access Certificate, but it still has to meet the accessibility requirements of the building regulations. Level access and wider doorways are much simpler to design in now than to add later.
Getting the right answer early
If you’re planning a granny flat or thinking about converting your home into two units, it’s worth getting a clear answer on the building control and fire safety side before you commit to a design. BCE Consulting Engineers has secured more than 50 planning permissions and over 30 fire safety certificates for clients across Cork, and our engineers are members of Engineers Ireland. Get in touch and we’ll talk you through what’s involved before work starts.
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