What are your legal rights if a neighbour decides to build up and block direct sunlight from hitting your solar panels?

It's a problem central Adelaide resident Jo Thomas was forced to confront when she learned a developer had plans to build a four-storey building next door.

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Dr Thomas is a medical doctor who lives in a small, medium density development called Christie Walk, which has 27 dwellings and a community garden.

"The emphasis is on nature and people-friendly urban development," she said.

"The plans were for a four-storey, very much box-like construction of apartments, on our boundary.

"The development had a big impact for us. For me personally it was going to throw quite a lot of shadow over my photovoltaic solar collectors and solar hot water system.

"But also for other residents, it was going to dramatically overshadow the community garden."

The neighbour's development was going to cast a significant shadow. ( Supplied: Julian Rutt )

Dr Thomas got expert help to have shadow diagrams drawn up.

"It was going to have a big impact on the amount of electricity I could generate from my photovoltaics ... there was at least a 35 per cent reduction in production. But for our solar hot water it was something in excess of 85 per cent," she said.

Going before the court

By making submissions about the potential impact to her local council during the public consultation process, Dr Thomas had her voice heard by the council's development assessment panel.

At this point, the council stepped in and stopped the development — but the developer appealed to the South Australia Environment Resources and Development Court.

This meant Dr Thomas was required to appear as witness before the court, in proceedings between the developer and the council.

Fortunately for Dr Thomas, the court found in the council's favour.

Jo Thomas lives at Christie Walk in Adelaide — a 'people-friendly' urban development. ( Supplied: Jo Thomas )

"There were many, many reasons given ... but basically the solar access was an issue, not just to the community garden, but to our solar technology and our hot water," she said.

"So all those things were considered, as well as the scale and bulk of the building ... which was considered incongruous with the rest of the streetscape."

But that wasn't the end of the story, as the developer has submitted more plans for the land.

"It's still a four-storey apartment block," Dr Thomas said.

"However ... the concessions that we have are that the top story is now stepped back on the eastern and the southern boundaries, and it really gives us a lot more protection of our photovoltaics and our solar hot water.

"Unfortunately, the big casualty for Christie Walk is the community garden, which is still going to be heavily overshadowed."

Many residents at Christie Walk contribute to the development's community garden. ( Supplied: Jo Thomas )

What rights exist around access to light?

As of April 2017, 1.6 million properties around Australia had photovoltaic solar panels — and new figures from the Australian Photovoltaic Institute show the country's solar power capacity is expected to double over the next few years.

So what does Dr Thomas's case mean for the rest of the country's solar panel owners? Do they have a right to sunlight without overshadowing?

Peter Clarke, a lawyer with Sydney firm Hones Lawyers, said that under many local government planning guidelines and development controls, there is a requirement that private open living spaces receive a certain amount of direct sunlight per day, with a minimum requirement on the winter solstice, June 21.

"Most jurisdictions require about three hours of sunlight into a private open living space," he told the Law Report after writing an article for Sanctuary magazine.

But the rules around access to sunlight for solar panels are much murkier.

A number of Australian jurisdictions have best practice guidelines and development controls that are set out as objectives.

But in terms of an actual right enshrined in law, nothing of the kind really exists.

"It's all a question of assessing how much of your roof used to get sunlight," Mr Clarke said.

"You can't exactly exercise your rights per se, but you may have grounds to seek a remedy. And that in itself raises a whole bunch of challenges."

Different councils, and even different areas within those council areas, have different rules.

The higher the density of an area, the less protection there is.

For example, someone living in a one-storey residence and surrounded on three sides by 40-storey buildings can't argue that the property on the fourth parameter can't go up, because they have solar panels that would be overshadowed.

A zoning conundrum

Mr Clarke said problems tend to arise when a zone for low-density housing, like one or two-storey detached dwellings, butts up against a zone for higher-density residential dwellings, like apartments — "particularly if it's on your northern boundary".

Mr Clarke said around the country, cases of this type have gone before the courts.

"Quite frequently, people have little recourse when an issue of overshadowing arises from buildings," he said.

"And in some states, no recourse whatsoever, if that overshadowing is caused by trees that haven't been trimmed, for example."

There are cases in which developments that would have overshadowed solar panels have been halted.

But for the most part these have been complicated matters, involving a decision that was made for reasons beyond merely a simple "right to light" for solar panels.

In one Victorian case, the City of Melbourne v Chen, a development was proposed that would have stopped sunlight from hitting an existing dwelling with 14 solar panels on the roof.

Density can be a factor when courts consider matters about light access for solar panels. ( Getty Images: Daniel Schoenen/LOOK-foto )

"One of the [proposed] units would have partially overshadowed the solar array on the winter equinox for quite a considerable period ... with an estimated total loss of about 50 per cent to 70 per cent, which is quite a lot," Mr Clarke said.

"That would be sufficient to render [the] rooftop array not worth keeping."

In that particular instance, the dispute went to the Victorian Civil and Administrative Tribunal (VCAT).

"On the balance of a number of factors ... it was held by the VCAT that [the project] was not an appropriate development to allow, and no permit to construct that development was granted," Mr Clarke said.

The tribunal member considered the development's impact on solar panels in making her decision — but her primary concern in denying the development was the fact that it would be "visually intrusive in the backyard realm" for neighbouring properties.

Helpful hint: Think hard about panel placement

Mr Clarke said a common factor discussed in cases about access to light for solar arrays was whether the solar panels were appropriately positioned in the first place.

"There have been cases that have mentioned it ... but it hasn't been a determinative point upon which granting or withholding of approval to build has been allowed," he said.

In the City of Sydney Council, for example, homeowners placing solar panels on a roof must ensure they take future developments into account.

"If you put your panels in a bad position with regard to what may go on in neighbouring properties, then the court will actually consider it," Mr Clarke said.

"But in the past, they've decided that it has not been enough of a determinative factor and they've permitted development to overshadow poorly placed solar panels."