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Do I need permission to extend my period home?

Q I have just bought a period house. Is it possible to extend without needing planning permission? If so, by how much and what can I do?

A The good news is that there are many exemptions that allow you to extend and alter your home without planning permission, such as attic conversions, extensions to the rear or even a new front porch.

If the original house has not been extended before, you can build an extension of up to 40 sqm to the rear. For a semi-detached house, any extension to the first-floor area can't be greater than 12 sqm. So, for a typical semi-d, you could build a ground-floor extension of 28 sqm with 12 sqm on the floor above. In practical terms, this should provide you with additional living space on the ground floor and an extra bedroom or bathroom upstairs.

But if the house already has an extension (built after October 1, 1964 and including any extensions where planning permission has been previously obtained), the total area of all the combined extensions mustn't be more than 40 sqm. This means that if the house already has a kitchen extension of, say, 10 sqm built after October 1964, then -subject to conditions - you can build on another 30 sqm to the rear without planning permission.

Think about height too. Any new extension you plan should not exceed the height of your existing house. If you want a flat roof on the new extension, its height shouldn't extend above the eaves level of the existing house. If the new extension is to have a pitched roof, then the height of that shouldn't be higher than the existing ridge-line. And then there is the issue of overlooking. Any new windows at ground level should be a minimum of one metre from the boundary they face and any windows on the first floor should be at least 11 metres from the boundary they face.

In terms of the garden, your new extension shouldn't reduce the "private outdoor space", ie, your back garden, to less than 25 sqm. But be warned, any garden sheds you already have or intend to add are not included in "private outdoor space", so this should be interpreted as 25 sqm plus whatever space the shed may need.

Many people are choosing to extend upwards into the attic space. Attic conversions generally do not require planning permission, nor are they counted in the 40 sqm exemption limit. Velux roof-lights to the rear generally don't require planning permission either, however, dormer windows and roof-lights to the front will require planning approval.

You can build a 2 sqm porch to the front of your house without applying for planning permission as long as the new porch is at least two metres from the front boundary and no more than three metres high for a flat roof and four metres high for a pitched roof.

You've bought a period house so you need to check that it isn't a protected structure - one that the planning authority considers to be of special interest from an architectural, historical, archaeological, artistic, cultural, scientific, social or technical point of view. Some, but by no means all, period houses are protected structures, so check the register of protected structures with your local authority before you buy or extend your home. If it is protected, these planning permission exemptions don't apply and you need to consult your local authority on any changes.

All that said, my advice is don't let planning permission exemptions dictate what you can and should do. First things first, appoint a RIAI-registered architect to consider and advise on all design options. An architect will explore a range of options, advise on planning implications and find the most economic and appropriate way to alter, extend and transform your new home.

Patrick Gilsenan runs an architectural practice, PG Architects, delivering domestic, commercial and community-based projects; pgarchitects.ie

Sunday Independent