As the snail grows, so does its calcium carbonate shell. The shell grows additively, by the addition of new calcium carbonate, which is secreted by glands located in the snail's mantle. The new material is added to the edge of the shell aperture (the opening of the shell). Therefore the center of the shell's spiral was made when the snail was younger, and the outer part when the snail was older. The adult bears a hard, thin calcareous shell 25–40 mm in diameter and 25–35 mm high, with four or five whorls. The shell is somewhat variable in colour and shade but is generally dark brown or chestnut with yellow stripes, flecks, or streaks. When the snail reaches full adult size, it may build a thickened lip around the shell aperture. At this point the snail stops growing, and begins reproducing.

   A snail's shell forms a logarithmic spiral. Most snail shells are right-handed.

 

   The body is soft and slimy, brownish-grey, and is retracted entirely into the shell when the animal is inactive or threatened. During dry and cold weather, the aperture of the shell is sealed with a thin membrane of dried mucus which is known as an epiphragm, which helps the snail retain moisture. The resultant quiescent periods are known as aestivation and hibernation respectively. When hibernating, Helix aspersa avoids ice formation by altering the osmotic components of its blood (or haemolymph), and can survive temperatures as low as -5°C. During aestivation, the mantle collar has the unique ability to change its permeability to water. In combination with an osmoregulatory mechanism similar to that seen during hibernation this allows Helix aspersa to survive several months of aestivation.

During times of activity the head and foot emerge. The head bears four tentacles, the upper two of which have eye-like light sensors, and the lower two of which are smaller, tactile and olfactory sensory structures. The tentacles can be retracted into the head. The mouth is located beneath the tentacles, and contains a chitinous radula which the snail uses to scrape and manipulate food particles.