Extending downwards is a popular tactic with high value properties and we regular read about the opposition to such schemes, not least because of the disruption that they cause. An increasing issue however is how they can impact on the future development rights of neighbours.
This particularly applies to those designs which do not use independent underpinning to retain the strip foundation system, instead (in layman’s terms) they effectively put a concrete box under the existing walls. The issue is that under the party wall legislation the neighbour typically owns 50% of the party wall that is being re-supported by a new type of foundation system. Depending on the design involved they might find ‘their half’ is supported on the neighbour’s new substructure instead of a new shared foundation. This is not always appreciated or picked up.
The issue might be fine (assuming that everything has been done properly) until the neighbour also wants to put a basement in – when they can find that the first developers design seriously constrains what they can do with their own property – leading to a party wall dispute (as the first development has compromised the future rights of the neighbour).
It can only be a matter of time before a serious case goes to court, particularly as owners of such properties tend to have ambitious plans and deep pockets. Some believe that the Party Wall legislation is not strong enough to deal with this, hence the ‘Subterranean Development Bill’ which had its first reading in the House of Lords in June.
The main practical challenge for developers is however the importance of avoiding delays to projects by ensuring that ‘Special Foundations’ are granted express permission from the neighbours themselves (and not just their surveyor). This reinforces the need for early appointment of Party Wall surveyors with experience of these developments and specialist basement developers.