Travel distances in Approved Document B (Table 2.1) explained

The idea behind every number here is simple. Anyone who meets a fire inside a building should be able to turn away from it and escape safely. Travel distance is how far they walk to do that: from any point on the storey to the nearest storey exit.

It's measured along the shortest real route. Where there's fixed seating, that means the centre line of the gangways. And if the route includes a stair, you measure along the pitch line.

Every limit in Table 2.1 comes as a pair. Take an office. Where escape is only possible in one direction, the limit is 18 metres.

Where people can escape in more than one direction, it's 45 metres. That longer limit applies to the nearest exit only. Other exits can be further away.

And a single escape route is generally only acceptable for a storey with no more than 60 people.

Here are the limits for buildings where people sleep, work, shop or gather. Notice the pattern. Where people may be asleep, or may need help to escape, the limits are much shorter: 9 metres one way, and 18 with alternatives.

That covers care settings, bedrooms, and buildings primarily for disabled people. Bedroom corridors, and the rest of a hotel or hostel, allow up to 35. Offices, shops and general assembly areas allow 18 and 45.

And areas with seating in rows sit in between, at 15 and 32.

Industrial and storage buildings get the longest everyday limits: 25 metres one way, and 45 with alternatives, but only where the hazard is normal. If the hazard is higher, those drop sharply, to 12 and 25. Places of special fire hazard, such as a boiler room, are limited to 9 and 18 within the room itself.

Plant rooms and rooftop plant have their own set, rising to 60 and 100 metres where the escape route runs in the open air.

Most escape routes start with no choice at all, like a single door out of a room. That’s fine, as long as three checks pass. Here’s an office.

From the start point, C, you walk twelve metres to B, where the routes split. First check: the two routes must be at least forty-five degrees apart. Here they’re a hundred degrees apart, so that passes.

Second: the dead-end leg, C to B, must be within the one-direction limit. For an office that’s eighteen metres. Twelve metres passes.

Third: the walk to the nearer exit, through B, must be within the more-than-one-direction limit of forty-five metres. Via exit A it’s thirty-seven metres. That passes too.

Early in a design, you often don't know where the partitions, desks or shelving will go, so you can't trace the real walking route. Table 2.1 has an answer. Measure the direct distance instead, in a straight line, and assume it's two-thirds of the real travel distance.

So in an office, a 45 metre travel limit becomes 30 metres direct, and 18 metres becomes 12.

So what makes a hazard higher? Note 3 to the table gives the list, for industrial and storage buildings handling significant amounts of hazardous materials. It covers gases under pressure, things that react with air or water, corrosives, oxidising agents, and substances that can catch fire on their own, give off heat as they break down, or spread fire by flowing.

Solids with a flash point under 120 degrees count too. And one entry surprises people: any liquid with a flash point below 65 degrees. The guidance names whisky and other alcoholic liquor specifically.