Building directly on your boundary is sometimes possible, for example certain garden walls, but for habitable structures it usually depends on planning rules, building regulations fire-safety separation requirements, and whether the boundary is shared with a neighbouring structure. It is not a simple yes or no and should be checked with the local authority and a designer before proceeding.
There is no blanket prohibition on building on a boundary in Ireland, but whether it is permitted in a given case depends on several separate considerations. Some structures, such as boundary walls and fences up to certain heights, are exempted development under the Planning and Development Regulations 2001 (as amended), Schedule 2, and can generally sit on the boundary line itself, subject to the specific limits in the regulations.
For a house, extension or outbuilding, building right up to or on the boundary raises additional issues: Building Regulations fire safety requirements (Part B) impose restrictions on openings, wall construction and roof coverings close to a boundary to limit fire spread between properties, and planning permission (where required) will assess the impact on the neighbouring property, including privacy and overshadowing. Some older terraced and urban buildings do share a party wall directly on the boundary, but new development of this kind is closely scrutinised.
Because building on or very close to the boundary affects a neighbouring owner's property, it is good practice, and often necessary, to discuss the proposal with them in advance, and to confirm the exact boundary location with a survey before finalising the design. A registered architect, engineer or planning consultant can advise on what is achievable at a given boundary.