Plans to extend the front facade of a listed Dublin landmark, Iveagh House on St Stephen's Green, will come as a surprise to conservationists.

Unlike other protected structures, Iveagh House is excluded from the planning regulations because of the security-sensitive nature of its function as headquarters of the Department of Foreign Affairs.

However, the Commissioners of Public Works have to give notice of their proposal to build a second-floor extension to the west side of the exterior.

The extension is required for the provision of an eight-person, wheelchair-accessible passenger lift to service the building, while the additional 656 sq ft being provided at second-floor level will be used for toilets displaced by the new lift.

The extension will be finished in Portland stone to match the original building. The lift will be located between the gable of Iveagh House and the adjacent building, a guest house called Stauntons on the Green.

Donated to the State by the Guinness family in 1939, Iveagh House had been used as a town house by Edward Cecil Guinness, founder of the brewery. It was designed by Richard Castle in 1736 and has a stunning interior, including a handsome staircase and ballroom.

As the office of the Minister for Foreign Affairs, it is frequently the venue for meetings and receptions for visiting foreign government delegations.

The building has always been subject to rigorous security and access to some areas is off-limits to members of the public.

Architectural plans for the proposed extension to the building are on display at the Project Management Services of the OPW on St Stephen's Green.

"This, in our opinion, is the only location within the old house where lift access can be provided, without affecting structure or finishes of historic merit," according to the OPW.

While An Taisce believes the issue of certain buildings remaining outside the planning system does need to be raised further, it says it has confidence in the quality of work carried out by the OPW with respect to State property.

"We are encouraged by the quality of the architecture work and the planning on existing projects. The recent Dβil extension was very well done," commented Ian Lumley, heritage officer at An Taisce.

Under Part 13 of the Local Government (Planning and Development) Regulations 1994, provision is made to exempt certain buildings from the planning regulations.

These include buildings where business is carried for Uachtarβn na h-╔ireann, Dβil ╔ireann, Seanad ╔ireann, the Department of the Taoiseach, the Office of the Tβnaiste, the Department of Defence, the Department of Foreign Affairs, the Department of Justice, the Office of the Attorney General, the Chief State Solicitor's Office and the Office of the Director of Public Prosecutions.

Provision is also made in respect of Garda stations or other structures used in connection with Garda operations, prisons or places of detention, courthouses, barracks, or other premises including airfields and naval yards used for the purposes of, or in connection with, the operations of the Defence Forces.

An earlier phase of construction work at Iveagh House is continuing. The proposed extension will be set back to reduce its impact on St Stephen's Green.