We want to build a separate granny flat. Where do we start?

We are looking into what options we would have to do up an existing four-bedroom semi-detached house along with building a granny flat. Ideally the granny flat would be a separate unit. There is enough room for the build. The existing house will need to be gutted and redone.

At the moment, there is a large separate garage to the side of the house. This is where we would like to build the additional housing unit. There is also a large garden to the back. Where do we start? Planning permission? Any advice would be appreciated.

The most important part of this question which will impact most on how you approach this project is the requirement of the granny flat. My understanding is that a unit of this nature must be interconnected with the main part of the dwelling to be considered a granny flat. Otherwise, it will be considered a second housing unit on the site and this is much more complicated from a planning perspective. This will obviously impact the design of your planned refurbishment as your plan to include the garage which is “separate” from the house may not be an option.

If the plan is to extend the house, you can extend up to 40sq m (430sq ft) and this would be exempt from planning permission (subject to certain exemption conditions) and you could look at this option. If your plan is for an extension larger than 40sq m, you will need planning permission.

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Substantial refurbishment

You have mentioned that you plan a substantial refurbishment of the house also. Following changes in building regulation standards late last year, and depending on how significant your refurbishment proposals are, including the age of the house, you may be expected to upgrade the entirety of the house to reach a minimum building energy rating of B2. This is likely to dictate a minimum specification in terms of amounts of insulation, type of windows, airtightness and the type of heating and ventilation systems the house needs.

Considering all of the above, it would seem that your starting point is to engage an appropriately qualified consultant to review and discuss your options with you, to prepare a design which best suits your needs and requirements and obtain any statutory approvals if required.

You may need to obtain a report from your local BER assessor to advise on the levels of insulation, standard of windows, heating systems etc, and this will have an impact on the cost of the refurbishment of the dwelling. Your appointed consultant will need to incorporate any advice from the BER assessor into the design and it is important to bottom this out in advance of starting any work on site so that specifications, budgets and the building contractor can account for this. – Aidan McDonald

Aidan McDonald is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland scsi.ie