The 31-year-old former One Direction singer bought the first of two houses in north London for Stg8.8 million ($A18.1 million) in 2019, added a second adjoining home for Stg4 million ($A8.2 million), and in 2024 purchased a third property nearby, according to documents seen by Britain's The Daily Telegraph newspaper.
Styles is now facing claims his proposal to join the two could obstruct natural light from surrounding houses.
He was granted permission in April to redevelop the site on the condition wildlife habitats, including hedgehog and bat boxes, were installed.
Under the proposals, the star intends to build what has been described as a "monster mansion" featuring a master suite with four dressing rooms, a gallery, drawing room, TV room, basement cinema, pool, sauna, gym and landscaped garden with a water feature.
But some local residents have raised concerns.
"It's sad living next to a building site all the time. This road didn't used to be like this," one neighbour told The Telegraph.
"All the houses are now constantly being bought by millionaires and redone.
"There's another house down at the bottom of the road, which I'm sure will be bought up and renovated like the others.
"There's also unfortunately less community among the neighbours than there was since people have moved on from the area."
Two neighbours have lodged formal complaints to the council over Styles' plans, warning their legal rights to light could be affected.
"My client is also deeply concerned about their private legal Rights to Light that might be infringed by virtue of the implementation of the proposal or a similar scheme of this nature," representatives for one homeowner wrote.
Another complaint said: "The building will be much larger in scale to all existing properties on the road, which are two stories, therefore out of scale."
In planning submissions, Styles' representatives said the proposed development "has been carefully crafted to bring the building back as one of the great Hampstead houses".
"It proposes to amalgamate the two houses, along with internal and alterations, extensions to the second, ground and lower ground floors, and extensive landscaping in order to reinstate and provide substantial enhancement to the Grade II listed building."
They added meetings had been held with residents who raised questions about daylight and construction disruption.
A daylight and sunlight study was subsequently commissioned, concluding there would be "a negligible impact on neighbouring properties in terms of daylight and sunlight".
Styles aims for the development to be completed by October 2027.
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