Eight days after stepping down as secretary of state in 2013, Hillary Clinton set up ZFS Holdings at CTC’s offices in Wilmington. A spokesman said it was to manage her book and speaking income. Photograph: Matt Rourke/AP

This squat, yellow brick office building just north of Wilmington’s rundown downtown is the registered address of more than 285,000 companies. That’s more than any other known address in the world, and 15 times more than the 18,000 registered in Ugland House, a five-storey building in the Cayman Islands that Barack Obama called “either the biggest building in the world, or the biggest tax scam on record”.

Officially, 1209 North Orange is home to Apple, American Airlines, Coca-Cola, Walmart and dozens of other companies in the Fortune 500 list of America’s biggest companies. Being registered in Delaware lets companies take advantage of strict corporate secrecy rules, business-friendly courts and the “Delaware loophole”, which can allow companies to legally shift earnings from other states to Delaware, where they are not taxed on non-physical incomes generated outside of the state.

The loophole is said to have cost other states more than $9bn in lost taxes over the past decade and led to Delaware to be described as “one of the world’s biggest havens for tax avoidance and evasion”.

But it’s not just big corporations that have chosen to make 1209 North Orange their official home.

Both the leading candidates for president – Hillary Clinton and Donald Trump – have companies registered at 1209 North Orange, and have refused to explain why.

Clinton, who has repeatedly promised that as president she will crack down on “outrageous tax havens and loopholes that super-rich people across the world are exploiting in Panama and elsewhere”, collected more than $16m in public speaking fees and book royalties in 2014 through the doors of 1209, according to the Clintons’ tax return.

Just eight days after stepping down as secretary of state in February 2013, Clinton registered ZFS Holdings LLC at CTC’s offices. Bill Clinton set up WJC LLC, a vehicle to collect his consultation fees, at the same address in 2008.

A spokesman for Clinton said: “ZFS was set up when Secretary Clinton left the State Department as an entity to manage her book and speaking income. No federal, state, or local taxes were saved by the Clintons as a result of this structure.”

The Clintons’ companies share the office with several of Trump’s companies. They include Trump International Management Corp and several companies that form part of Hudson Waterfront Associates, a Trump partnership to develop more than $1bn worth of luxury condos on the west side of Manhattan.

Of the 515 companies on Trump’s official Federal Election Commission (FEC) filing, 378 are registered in Delaware, he revealed, after being questioned by the Guardian about why so many of his New York-based companies are incorporated in Delaware.

He said he asked his staff to find out how many entities he has in Delaware. “I figured they’d maybe say two or three, right?” Trump said at a rally in Harrington, Delaware, on Friday. “We have 378 entities registered in the state of Delaware, meaning I pay you a lot of money, folks. I don’t feel at all guilty, OK?”

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Among them are 40 Wall Street Corporation, Trump’s 72-storey downtown tower that was the tallest building in the world for two months in 1930, and the Trump Carousel in Central Park.

The Trump campaign did not respond to questions about whether Trump was using Delaware in order to avoid taxes in New York.

It is not unusual for rich individuals and companies to register their business in Delaware due to the ease of company formation in the state, but the Clintons’ and Trump’s companies in the state are likely to come under greater scrutiny as the US presidential primary roadshow rolls into the state on Tuesday. A poll by research firm Gravis Marketing last week showed Trump had a 37-point lead over John Kasich; Clinton polled 45%, ahead of Sanders on 38% in the same poll.

A report by the Institute on Taxation and Economic Policy, titled Delaware: An Onshore Tax Haven, said the state’s tax code made it “a magnet for people looking to create anonymous shell companies, which individuals and corporations can use to evade an inestimable amount in federal and foreign taxes”.

Several accounting experts said there are many legitimate reasons why US and foreign companies incorporate in Delaware, particularly because of its highly respected Court of Chancery and business-friendly state government. The process of setting up a company in the state can be completed in just a few hours and requires less paperwork than registering for a library card in the state. There are more than 1m companies registered in the state – more than Delaware’s population of 935,000.

In the US presidential election, Clinton’s rival Bernie Sanders has led the charge to counter corporate greed, and highlighted the tax havens revealed by the Panama Papers as evidence that “the wealthiest people and largest corporations must start paying their fair share of taxes”.

Clinton has called offshore tax havens “a perversion” of the legal code, and Obama called for reform of the international system earlier this month. Even Trump has said he supports raising taxes on the wealthiest Americans, “including myself”, though his tax plan offers cuts.

The Guardian Media Group, owner of theguardian.com, is registered in Dover, Delaware. “Guardian Media Group has business operations in the UK, US and Australia,” a Guardian spokesperson said. “The group’s assets are held entirely by companies in these countries and are fully subject to prevailing tax laws and regulations. The group also has a UK endowment fund which holds a mixture of UK and non-UK assets and is fully subject to UK tax laws and regulations.”