Eoghan Murphy (Dublin Bay South, Fine Gael) | Oireachtas source

Substantial change is defined as works that consist of a permanent extension that increases the floor area by 25% or where three of four things happen, which are raising the BER by two points; adding an extra room; making a change in the internal layout of the home; or an adaptation to allow for disability access. If landlords do that they can put in an application to the RTB, signed off by a registered professional such as an architect or surveyor as to the extent of the work, and they can then increase the rent by more than 4%. The work would necessitate a person vacating a property so it is something significant.