People in Mendip have worse access to green space than most people in the South West, new figures suggest.
The Health Foundation said the Government must prioritise providing access to green spaces in its plan to build more homes, adding "innovative action" is needed to improve access in deprived neighbourhoods.
The Department for Environment Food and Rural Affairs estimates 20,711 of the 54,497 households in Mendip do not have an accessible green space of two hectares or larger within one kilometre of their house – accounting for 38% of homes.
It suggests people in Mendip have worse access than most people in the South West, where 30% of all households do not have access to green spaces.
Gwen Nightingale, assistant director of the Healthy Lives team at the Health Foundation, said studies show people living in areas with more access to green space tend to have better general health, mental wellbeing, and higher life expectancies than those with limited access.
She added those living in the most deprived neighbourhoods are four times more likely to live in areas with the least access to green space, saying "innovative action" is needed to improve inequalities.
She said: "For new homes and neighbourhoods, the Government must prioritise providing access to green space within their pledge to create new towns and build 1.5 million new homes."
Across England, 78% of households have a green space of at least two hectares within a kilometre.
Mike Childs, head of policy at Friends of the Earth, said: "While it is good news that the majority of people have reasonable access to green space, this still means at least one in five people don't, and previous research has shown that this 20% of the population is disproportionately made up by people of colour."
He added: "Since Covid there has been a greater awareness of the importance of green spaces for both physical and mental health, as well as the critical role they play in cooling urban heat.
"The goal must be to ensure everyone has access to green space, nature and the benefits they bring."
Defra's statistics also use other standards from Natural England to measure access to green space, which vary in distance and size.
In Mendip, 9% of households meet the doorstep standard, meaning they have an accessible green space of at least 0.5 hectares within 200 metres.
Meanwhile, 3% meet the local standard – a green space of at least two hectares within a five minute walk from home.
And 28% of households meet the neighbourhood standard, as they had a green space of at least 10 hectares within one kilometre or a 15 minute walk from home.
A Defra spokesperson said: "Our countryside and green spaces are a source of great national pride. But too many across the country are left without access to the great outdoors.
They added the Government will increase access to nature by creating nine new national river walks and planting three new national forests, while a new community right-to-buy will "empower" communities to create new parks and green spaces.