The construction of Churchill Barrier No. 4 in 1943, a causeway in the Orkney Islands of Scotland, has led to the accretion of a substantial beach-dune system, the so-called ‘Ayre of Cara’. As a result of the blocking of tidal flows through Water Sound into Scapa Flow, the deposits have accumulated at an average rate of 7.7 × 103 m3 a−1, which is manifest at the land surface as an average lateral advance rate of 1–6 m a−1. The Ayre of Cara is the only accreting dune system in the Orkney Islands and the rate of lateral growth of the beach is one of the fastest in Britain. Since the causeway, one of four built in World War II to defend the British naval fleet, was not intended to ‘claim’ land, the fundamental question raised is who owns the sand that has accumulated? This question is especially pertinent given that the materials deposited have, at one part of the site, been extracted for commercial gain under license granted by the local authority. A series of theoretical ownership scenarios, which pertain to British legal systems, demonstrate that the situation is complex and that a definitive answer is not yet possible as there has not been a case from this specific site brought before a court, or even from a similar site. However, were property disputes to arise in comparable situations in the future, this paper details for the first time the potential level of confusion and complexity that is likely to arise. The findings of this work also have similar relevance for similar situations in other legal systems in the world.