Following the submission of a planning application for the proposed conversion of a disused barn at Hazel Gill, Mallerstang, Cumbria, into a dwelling, Greenlane Archaeology was commissioned to carry out a heritage assessment, following a request for additional information from the Yorkshire Dales National Park. This examined the historical development of the building based on documentary sources and an archaeological building recording, and was carried out by Greenlane Archaeology in October 2022. The barn is shown from early mapping to have existed from at least 1839, although the farm at Hazel Gill, to which it belonged, was in existence from at least 1672. The maps show that it had definitely taken its present form by the late 19th century, although the earlier mapping is not of a scale sufficient to show some detail. The building recording was able to identify three phases of development within the building, from its initial construction in the late 18th or, more likely early 19th century, as a field barn in the bank barn style, with accommodation for cattle and feed on the lower floor, and space for the storage of hay on the upper floor. A channel originally existed along the west side, which a ventilation slot and window made use of, while the main entrance was accessed via a ramp across it. This originally arrangement was modified in the late 19th or early 20th century, the channel along the west side was filled in, blocking the slot and window, and some modifications were made to the stalls in the byre. More recently it has had new doors and window casements, and possibly also a new roof and floor, but has since fallen into disuse and seen some damage caused by water ingress. The building represents a relatively typical example of such a structure, albeit in relatively good condition. However, the proposals would have some negative impacts on the original fabric through the insertion of new windows and the removal of the original stalls. In the former case, alternative suggestions have been made and, in the latter, the stalls are in poor condition and have now been fully recorded as part of this project. Despite being located within an area of potentially medieval field systems, it is considered unlikely that any below-ground archaeological remains will be impacted upon groundworks associated with the development, but this could be mitigated by an archaeological watching brief if considered necessary.