It's open on most Saturdays for two types of tour: a guided tour with an expert guide, or a self-guided tour with a pair of headphones (which is a bit cheaper).
The guided tour takes approximately 75 minutes and follows the processional route taken by Her Majesty the Queen when she performs the State Opening of Parliament. After meeting your guide in the 1,000-year-old Westminster Hall, you'll be taken to see the Norman Porch, the Robing Room, the Royal Gallery (one of the most spectacular rooms in Parliament -- it almost rivals the rooms at Buckingham Palace!) and Prince's Chamber, before finally entering the House of Lords. Then you'll head into the Central Lobby, Member's Lobby and House of Commons.
The moment you walk into the Commons for the first time will be a special thrill (especially for a British visitor, because they see this room on the news every night). You'll walk around the back of the Speaker's Chair and stand in the MPs benches, before heading out into St. Stephen's Hall. Then the tour will finish back where it began, in Westminster Hall -- the medieval room that witnessed the trials of Guy Fawkes and Charles I.
The self-guided tour is a bit different because you get to walk around at your own pace, punching the numbers into your headphones as you enter each room, to hear a bit of history. You still have to stick to the same route as the guided tour, though -- but you can take a bit longer.
Craig has tried both of these tours himself, plus the Summer Opening (which opens every day of the week), and recommends a 'Saturday Guided Tour' for first-time tourists because it's slightly less crowded than the Summer Opening, giving your guide more time to describe the rooms in detail. The self-guided audio tour is better for people who've been there before. But have a read of his reviews before you go, because he's described them all on his London blog. Here's his one for the Saturday guided tour, and here's his one for the self-guided audio tour. You might like to read through his review of the Summer Opening as well.
If you enjoy this tour, then you might be interested in coming back during the week to see the MPs debating in the House of Commons. (Check out Craig's review of the House of Commons and House of Lords -- he's written reviews of the whole lot). You might even be tempted to get a ticket for Prime Minister's Questions.
If you really want to treat yourself, then how about combining this tour with an afternoon tea in Parliament?
Admin – “they have a ticket office opposite parliament (directly across the road from the victoria tower — the other end of the building to big ben). the ticket window is right next door to the jewel tower”
Guest – “Just to let you know .The ticket office has moved to portcullis house ,on the the embankment,opposite the statue of boudicia.”
Craig’s review – “I'm back at the Houses of Parliament today. I haven't been here for a while so I thought I'd better just check the politicians are behaving themselves. You know what they are like... if we don't keep our beady eyes on them they start getting drunk and having fist fights. You might be under the impression that you can only have a tour of parliament during the Summer O… continued”
Houses of Parliament tour with afternoon tea – Every Sat – Enjoy a Saturday tour of Parliament, and then enjoy an afternoon tea on the Terrace Pavilion overlooking the Thames.
Watch a case at the Royal Courts of Justice – Mon to Fri – Why not try something a bit different -- how about going to the Royal Courts of Justice and watching a real-life court case?
Watch a real-life court case at the Old Bailey – Mon to Fri – Watch the long arm of the law in action at the Old Bailey, where members of the public can sit in the gallery and watch a case.
Mansion House: The Lord Mayor’s Residence – Every Tue – Look inside the Lord Mayor of London's official residence at Mansion House and see the interior courtyard and famous Egyptian Hall.
Common Council meeting with the Lord Mayor – 18 Oct, 6 Dec 2018, 10 Jan, 7 Mar, 25 Apr, 23 May, 20 Jun, 18 Jul, 12 Sep, 10 Oct and 5 Dec 2019 – Attend a meeting of the Common Council at the Guildhall and see the Aldermen and Lord Mayor of London
Listen to a speech at Speakers’ Corner – Every Sun (around noon) – Visit Speakers' Corner on a Sunday lunchtime and hear orators discussing whatever they like in the name of free speech.
Listen to MPs in the Houses of Commons – 7 Jan to 18 Dec 2019 – Visit Parliament for free and watch MPs debating in the House of Commons and House of Lords.