Yes, house extensions in Ireland must comply with the Building Regulations regardless of whether they need planning permission. This includes structural safety, fire safety, ventilation, and energy performance requirements, and most extensions above a certain size require a commencement notice to be lodged before work starts.
Building Regulations apply to house extensions independently of planning permission. Even a small extension that qualifies as exempted development under the Planning and Development Regulations 2001 (as amended), Schedule 2, and therefore needs no planning permission, must still be built in compliance with the Building Regulations because it involves construction work covered by the Building Control Act 1990. This means matters such as structural stability, fire safety between the extension and the existing house, damp-proofing, ventilation, and thermal performance under Part L must all be addressed in the design and build.
In practice, this means an extension must be designed to meet the relevant Technical Guidance Documents, typically with input from a competent designer such as an architect or engineer, and constructed by a competent builder. Where a fire safety certificate or disability access certificate is required, for example for certain larger or more complex works, these are separate statutory certificates applied for through the local authority Building Control Section before construction. For most single-storey domestic extensions these certificates are usually not required, but this should be confirmed case by case.
A commencement notice generally has to be lodged with the local authority through the Building Control Management System (BCMS) before certain works start, giving statutory notice of construction so the works can be recorded and, where required, inspected. Whether a notice is needed depends on the type, scale and value of the works, so this should be checked directly with Building Control or a competent professional before work begins.