An area of 16.63 acres (6.73 ha) has been declared an SSSI for its unimproved neutral grassland of the type crested dogs-tail Cynosuras cristatus, common knapweed Centaurea nigra lowland hay meadow and pasture, and heath-grass Dantonia decumbens sub-community. Mill Meadows SSSI is comprised of five adjoining grassland units that overlie Claygate beds (sandy clay or loam) and London Clay, sloping to the north and seperated by old hedge lines, some with ditches. The site is bounded by housing, roads, a stream, semi-natural grassland, scrub and secondary woodland. Although the site lacks any rare species it supports a characteristic flora of a grassland type that is very much reduced within the Essex landscape due to development and agricultural improvement.

The grassland is neutral to acid and dry to wet in character. It is composed of a sward largely of red fescue Festuca rubra and common bent Agrostis cappilaris within which Devil's-bit scabious Succisa pratensis, tormentil Potentilla erecta and common knapweed Centaurea nigra are frequent. Locally frequent to abundant are burnet saxifrage Pimpinella saifraga, betony Stachys officinalis and field wood-rush Luzula campestris. Other species include harebell Campanula rotundifolia, yellow rattle Rhinanthus minor, common spotted orchid Dactylorhoza fuschii, common birds-foot-trfoil Lotus cornicultas and pignut Conopodium majus. To the north there are several flushed areas that support a similar flora to that already noted but with the additions of plants including sharp-flowered rush Juncas acutiflorus, angelica Angelica sylvestris, greater birds-foot-trefoil Lotus pendunculatus, marsh thistle Cirsium palustre, sneezwort Achillia ptarmica, cuckooflower Cardimine pratensis, ragged robin Lycnis flos-cuculi and common marsh-bedstraw Galium palustre.