Gideon Mantell originally estimated that Hylaeosaurus was about 7.6 metres (25 ft) long, or about half the size of the other two original dinosaurs, Iguanodon and Megalosaurus.
At the time, he modelled the animal after a modern lizard.
Modern estimates range up to 6 metres (20 ft) in length.
Gregory S. Paul in 2010 estimated the length at 5 metres (16 ft), the weight at 2 tonnes (2.0 long tons; 2.2 short tons).
Some estimates are considerably lower: in 2001 Darren Naish e.a.
gave a length of 3–4 metres (9.8–13.1 ft).

Many details about the build of Hylaeosaurus are unknown, especially if the material is strictly limited to the holotype.
Maidment gave two autapomorphies, unique derived traits: the scapula did not fuse with the coracoid, even when the animal was of a considerable size; there were three long spines on its shoulder.
Even these traits are not very distinctive: Mantell and Owen had attributed the lack of fusion to ontogeny and the total number of spines cannot be observed.
Hylaeosaurus is often styled as a fairly typical nodosaur, with rows of armour plating on the back and tail combined with a relatively long head, equipped with a beak used to crop low-lying vegetation.

In 2001 the skull and lower jaws remains were described by Kenneth Carpenter.
The damaged and shifted skull elements provided little information.
The quadrate is laterally bowed.
The quadratojugal has a high attachment point on the shaft of the quadrate.
A triangular postorbital horn was present.

The spines at the shoulder are curved to the rear, long, flattened, narrow and pointed.
Their underside shows a shallow trough.
The front spine is the longest at 42.5 centimetres; to the rear the spines become gradually shorter and wider.
A fourth spine, of about the same build but more forward-pointing, is present immediately behind the skull.
In 2013 Sven Sachs and Jahn Hornung suggested a configuration in which there were five lateral neck spines, the new German spine having a morphology adapted to fit in the third position.
Gideon Mantell originally estimated that Hylaeosaurus was about 7.6 metres (25 ft) long, or about half the size of the other two original dinosaurs, Iguanodon and Megalosaurus. At the time, he modelled the animal after a modern lizard. Modern estimates range up to 6 metres (20 ft) in length. Gregory S. Paul in 2010 estimated the length at 5 metres (16 ft), the weight at 2 tonnes (2.0 long tons; 2.2 short tons). Some estimates are considerably lower: in 2001 Darren Naish e.a. gave a length of 3–4 metres (9.8–13.1 ft).
Many details about the build of Hylaeosaurus are unknown, especially if the material is strictly limited to the holotype. Maidment gave two autapomorphies, unique derived traits: the scapula did not fuse with the coracoid, even when the animal was of a considerable size; there were three long spines on its shoulder. Even these traits are not very distinctive: Mantell and Owen had attributed the lack of fusion to ontogeny and the total number of spines cannot be observed. Hylaeosaurus is often styled as a fairly typical nodosaur, with rows of armour plating on the back and tail combined with a relatively long head, equipped with a beak used to crop low-lying vegetation.
In 2001 the skull and lower jaws remains were described by Kenneth Carpenter. The damaged and shifted skull elements provided little information. The quadrate is laterally bowed. The quadratojugal has a high attachment point on the shaft of the quadrate. A triangular postorbital horn was present.
The spines at the shoulder are curved to the rear, long, flattened, narrow and pointed. Their underside shows a shallow trough. The front spine is the longest at 42.5 centimetres; to the rear the spines become gradually shorter and wider. A fourth spine, of about the same build but more forward-pointing, is present immediately behind the skull. In 2013 Sven Sachs and Jahn Hornung suggested a configuration in which there were five lateral neck spines, the new German spine having a morphology adapted to fit in the third position.
Hylaeosaurus