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It sounds like you would need permission for what you have planned. You need to apply to Building Control in Belfast City Council for building regulations approval if you want to convert a roofspace or attic into:
If your conversion creates a new storey with a floor area in excess of 3 square metres, then you will need to submit a full plans application. This involves filling in the necessary forms, submitting two sets of drawings and specifications and the correct fee.
You also need to consider fire safety when you add a storey on to a property - these vary depending on whether the property is going from one to two storeys or from two to three, but essentially you need to make sure:
If you are going from two to three storeys, you must make sure that existing stairways are fire resisting, plus some doors will need to be self-closing; others will need to be fire resisting.
You will also need to think about sound - if it's going to be anything other than a storage area, the walls and floors between the new habitable room and other habitable areas will need sound insulation. You mention that it's a terraced house - if it is adjacent to habitable rooms in other buildings then sound testing will be required.
Even if the area is only ever to be used as a storage space, if you plan to install a fixed stairway rather than using the attic steps/ladder, you will need to apply to Building Control for approval.
You'll also have to comply with regulations on moisture, energy and ventilation - the walls and roof must keep out rain and damp and resist heat loss, as well as prevent condensation build up.
Personally, I think that all sounds like a lot of work and expense to add a storage room or an office to a terraced rental house, but it's up to you to balance that against the added value it would offer and the increased likelihood of attracting your ideal tenant.
I tend to think if you're going to go to all the trouble of a roofspace conversion, another bedroom (with an ensuite if possible) may be more likely to attract those young professionals.
If you have any questions about building control, planning permission or anything else related to property - whether you're a buy-to-let investor or a private buyer or owner - join the conversation over on Facebook, Twitter or Linked In, or drop me a line via email - all the links and addresses you need are in the top right corner of this page.