(See also Chapter 9, Section 9.5.4)
In recent years, there has been a significant growth in new basement development and extensions to existing basement accommodation. Basements can provide valuable additional accommodation for leisure or storage purposes. However, basements can be prone to ﬂooding. There are also a growing number of planning applications looking to maximise accommodation on restricted sites by proposing habitable basement accommodation sunk one full level below ground and extending up to 100% of the site area, often with a ground ﬂoor/roof garden on top.
It is the policy of Dublin City Council to discourage any significant underground or basement development or excavations below ground level of, or adjacent to, residential properties in Conservation Areas or properties which are listed on the Record of Protected Structures. Development of all basements or any above ground buildings for residential use below the estimated flood levels for flood zone areas ‘Zone A’ or ‘Zone B’ will not be permitted (Policy SI13).
In considering applications for basement developments, the planning authority will have regard to the following:
- The permissible size of a basement development to the rear of a property will be guided by the characteristics of the site. In the case of large sites, a basement development to the rear of a property generally should not exceed the footprint of the original building. In all cases, a basement development should generally not extend to more than 50% of the amenity/garden space
- Impact of proposal on future planting and mature development of vegetation and trees on the site
- Impact of the water table and/or any underground streams and sewers
- The basement development should provide an appropriate proportion of planted material to mitigate the reduction in the natural storm water inﬁltration capacity of the site and the use of Sustainable Urban Drainage Systems is recommended
- Measures should be taken by the applicant during demolition and construction works to ensure that the structural stability of the existing property, adjoining properties and critical infrastructure is maintained
- Adequate sunlight/daylight penetration will be required which will be inﬂuenced by site orientation and the size of site and any lightwell/courtyard; the planning authority may require a daylight analysis to be submitted as part of a planning application
- Adequate ventilation will be required; kitchens, bathrooms and utility areas should ideally be naturally ventilated
- Basements should be provided with a means of escape allowing access to a place of safety that provides access to the external ground level.