The origins of common land date back to feudal times. Back then the poorest, least productive soil in a parish was often designated as common land, and was available for parishioners to graze animals and/or cut turf and timber for fuel. Members of this community with these rights were known as commoners.
So for example Richmond Park is not a common, it's a park, and Blackheath isn't a park, it's a common. Blackheath is jointly owned by the Crown (to the north of the A2) and by the Lord of the Manor of Lewisham (to the south). Specifically it falls under the protection of the Metropolitan Commons Act of 1866 (it was added in 1871) which makes it a common not a park. Richmond Park has been royally owned for centuries and no common rights were ever granted to mere underlings.
Wading through official designations isn't fun, but fortunately Wikipedia has a List of common land in London in a clickably sortable format. It looks definitive because it includes over 100 commons including some impressively obscure ones, and several of the areas are listed to two decimal places.
According to Wikipedia London's largest common is
1) Mitcham Common(152 hectares)
This has been grazing land for millennia, the original oak woodland having been cleared by early Neolithic people. But the underlying river gravels mean the soil here is poor, hence grazing led to a preponderance of shrubby acid heathland. Those gravels eventually proved appealing to 19th century road builders, which along with early 20th century urbanisation led to the end of grazing and a fair amount of scrubby encroachment. Since 1891 its been overseen by the Mitcham Common Conservators, or what's left of it, and marvellously explorable it is too.
Except Mitcham Common actually has an area of 182 hectares according to the Conservators' website, which makes the Wikipedia list look potentially ropey. And if you dig into the page's edit history it turns out Mitcham Common was only added to the list by an unregistered account in 2022, then had its area tweaked six months later. It's all somewhat dubious.
Where, for example, is the far larger, far more obvious other common in the London borough of Merton?
1) Wimbledon Common(461 hectares)
This is much bigger, indeed at 1¾ square miles it's considerably larger than the City of London. It's also one of the first commons to be protected by specific Parliamentary legislation, this to fend off an attempt by the landowner to enclose part of the common for his own use and sell off a large chunk for housing. The Wimbledon and Putney Commons Act 1871 secured its future and placed the land under the custodianship of a board of conservators who still operate (and are holding elections in March, if you're interested). Wimbledon Common is fabulously sprawly, always worth a wander and kept inexplicably free from litter.
Except it's not all one common. Three other constituent parts make up its total acreage, the largest of which is Putney Heath which merges invisibly into Wimbledon Common from the north. That's 160 hectares, then there's the separate Putney Lower Common (13 hectares) and also the Richardson Evans Memorial Playing Fields (19 hectares), leaving Wimbledon Common proper with about 270 hectares. It's still enough to be comfortably the largest common in London but not as vast as the headline figure.
Wikipedia's List of common land in London was created in 2013 and derives explicitly from a biological survey of registered common lands in Greater London which was published by Defra in 2002. It's one of several reports compiled by the Rural Surveys Research Unit at the University of Wales in Aberystwyth, so is impressively detailed and authoritative. The researchers' sources were the official registers of common land maintained by London's local authorities - a legal requirement - and I'm glad they tracked down all 33 because that task would have been way beyond me.
The researchers found that...
• Greater London contains 122 separately identified commons.
• The majority of commons (53%) are between 5 and 50 hectares, while 38% are less than one hectare.
• The dominant cluster of commons falls in the area between Ealing, Paddington and Richmond.
• 17 commons are contiguous with other commons.
• The most popular land use is broadleaved woodland, followed by what's called "amenity grassland".
• Ickenham Marsh is an example of marshy grassland, Keston Common includes mire habitat and Rowley Green exhibits acidic flush.
• The boroughs with the most common land are Wandsworth, Hackney, Greenwich, Camden, Bromley and Richmond.
And in the researchers' supposedly authoritative list, London's largest common is
1) Hampstead Heath(145 hectares)
Hampstead Heath's never been much good for farming because it lies across a sandy ridge overlying clay. It faced its greatest threat in the 1860s when the landowner started flogging sand, digging brickfields and making plans to build avenues of mansions. Local people duly set up a Commons Preservation Society which kickstarted the whole legislation bandwagon, enshrining that ‘the Board shall forever keep the Heath open, unenclosed and unbuilt on'. It's still a magnificent green lung, the closest natural escape to central London, and remains an unenclosed undeveloped treasure.
Hampstead Heath's Wikipedia page states that it "contains the largest single area of common land in Greater London", which sounds promising for our purposes, except its sole reference turns out to be the research report I've already mentioned. With Wimbledon Common, Putney Heath and Mitcham Common all patently larger, we need something a bit more statistically convincing.
What I think's going on here is that some commons are registered and some aren't. The Commons Registration Act 1965 requires all common land to be definitively registered as such and Wimbledon Common isn't because it's not on Merton's statutory list. Ditto Mitcham Common, and ditto Putney Heath not being on Wandsworth's, but Hampstead Heath is on Camden's so it's definitively London's largest registered common.
However this dubious list of London's commons starts, this is how it continues...
» Hampstead Heath (Camden) 145 ha
» Hackney Marsh (Hackney/Waltham Forest) 136 ha
» Hayes Common (Bromley) 91 ha
» Blackheath (Lewisham/Greenwich) 86 ha
» Lambourne Common (Redbridge) 80 ha
» Clapham Common (Wandsworth/Lambeth) 78 ha
» Monken Hadley Common (Barnet) 74 ha
» Wormwood Scrubs (Hammersmith and Fulham) 73 ha
» Wandsworth Common (Wandsworth) 69 ha
» Woolwich Common (Greenwich) 60 ha
» Tooting Bec Common (Wandsworth/Lambeth) 58 ha
» Barnes Common/Barnes Green (Richmond) 50 ha
» Ham Common (Richmond) 49 ha
» Stanmore Common (Harrow) 48 ha
» Plumstead Common (Greenwich) 41 ha
» Tylers Common (Havering) 32 ha
» Bostall Heath (Greenwich) 30 ha
» Streatham Common (Lambeth) 24 ha
» Peckham Rye Common (Southwark) 23 ha
» Tooting Graveney Common (Wandsworth) 22 ha
n.b. "Figures specifying the areas of these commons cannot be regarded as accurate measures. More detailed mapping and calibrations made during the biological evaluation of the commons have indicated that errors can be of a significant order"
