When designing your development it is not unusual for there to be overhangs over the public highway (permanent, e.g. balconies or temporary, e.g. scaffolding). Whilst a licence from the Highway Authority can be secured, developers should be alive to the possibility of the airspace above the highway being within third party ownership. In this situation an easement or licence to overhang will also need to be negotiated with the third party. Without such an easement or licence you are at risk of a claim for trespass, potentially resulting in an injunction which would probably be concluded by payment of a premium for release of the injunction and grant of oversail rights.
You would be advised to carry out a Land Registry search at an early stage to establish whether the highway land is registered and the details of the owner. Even if the land is unregistered a cautious developer may consider indemnity insurance against the risk of a third party claim of ownership. A developer should also be mindful of the impact on plot sales of balconies which overhang third party land.