Why Boris Johnson's apartment renovations have landed him in the political doghouse

The UK Prime Minister is no stranger to backlash from across the political aisle.

London (CNN)British Prime Minister Boris Johnson will face a formal investigation into the funding of renovations to his Downing Street apartment, after the country's political spending watchdog said there are "reasonable grounds" to suspect that an offense may have occurred.

Johnson has been under pressure for several days to explain how he paid for the upgrade to the flat, with British news outlets reporting that the work cost around £200,000 ($280,000).
The saga was elevated last week when Dominic Cummings, Johnson's former closest ally, leveled accusations of "unethical, foolish, (and) possibly illegal" behavior against his old boss, claiming he discussed plans to have Conservative Party donors pay for the refurbishments.
    The opening of an investigation comes at a tense moment for the Prime Minister, whose party is preparing to contest local elections across the country next week.

      Where does Boris Johnson live?

      Compared with the Executive Residence at the White House or the various presidential palaces dotted around the world's capital cities, the British Prime Minister's London living quarters are relatively humble.
      Leaders would traditionally live in a flat above Number 10 Downing Street, but recent Prime Ministers have instead moved into the larger apartment next door, at Number 11 -- above the offices of the Chancellor of the Exchequer.
        Even so, the apartment is far from spacious. The buildings that line Downing Street consist of cramped offices and narrow corridors, and Prime Ministers are not afforded luxury in their official residence.
        US first lady Michelle Obama and Samantha Cameron, wife of British Prime Minister David Cameron, talk in the Camerons' private residence at Downing Street in 2011.
        Prime Ministers are given £30,000 ($41,000) of public money a year to renovate the apartment during their term, and it is not uncommon for them or their spouses to put their own stamp on the interior design.
        Tony Blair faced questions in 2002 over the cost of his revamp, which reportedly included adding a study. Pictures released during David Cameron's premiership showed a modern kitchen and open-plan living space, and Theresa May posed for a 2016 interview next to a fireplace, though it's not clear whether she made any significant changes to the interior design.
        Of course, British leaders are not confined to this small apartment all year. The far more spacious country house residence of Chequers sits in Buckinghamshire, a short drive from London, and that destination is often used to entertain foreign leaders or to provide the Prime Minister with an escape from Westminster.

        Why are Johnson's renovations controversial?

        Johnson has been under pressure for weeks to explain how he paid for the renovations to his apartment, which British news outlets have reported cost around £200,000 ($280,000).
        Those renovations were reportedly overseen by Johnson's fiancée, Carrie Symonds. But Cummings, who left his role late last year after amassing a reputation as the driving force behind the Prime Minister's Brexit policy and re-election, alleged in a blog post that Johnson planned to have Conservative Party donors pay for the upgrade.
        If that was the case, the loan would need to have been declared to the UK's Electoral Commission. Political donations and loans are tightly controlled in the United Kingdom, with loans of more than £7,500 ($10,400) logged and publicly revealed by the commission four times a year.
        "I told him I thought his plans to have donors secretly pay for the renovation were unethical, foolish, possibly illegal and almost certainly broke the rules on proper disclosure of political donations," Cummings wrote in an explosive post on Friday.
        Dominic Cummings, left, and Boris Johnson during their time together at Downing Street.