Yes, building without required planning permission, or failing to comply with an enforcement notice, is an offence under the Planning and Development Act 2000 and can result in prosecution and significant fines, and in serious cases imprisonment. Fines and penalties depend on the court, the seriousness of the breach, and whether the case is dealt with summarily or on indictment.
The Planning and Development Act 2000 creates criminal offences for carrying out unauthorised development and for failing to comply with an enforcement notice, and these offences can be prosecuted either summarily in the District Court or, for more serious cases, on indictment in the Circuit Court. A prosecution typically follows a period of unsuccessful enforcement engagement, meaning fines are usually a later-stage consequence rather than an immediate response to a first-time breach.
The scale of penalties depends on the level at which the case is prosecuted, with the District Court able to impose more limited fines and the Circuit Court able to impose substantially larger fines and, in the most serious cases, custodial sentences, along with ongoing daily fines for a continuing offence in some circumstances. Courts can also order remedial works, such as demolition or restoration, in addition to any financial penalty, and can require the offender to cover the local authority's legal and enforcement costs.
Because prosecution is a formal legal process, most cases only reach this stage where the owner has ignored a warning letter and an enforcement notice, refused to apply for retention, or had retention refused and still failed to comply. Engaging early with the planning authority and, where appropriate, applying for retention permission is the most effective way to avoid the risk of prosecution and fines.