StuG Ausf.A (Sd.Kfz.142) was based on the Pz.Kpfw.III Ausf.F. The new design replaced the turret and superstructure with a low superstructure mounting the 7.5cm gun offset to the right. The gunner's sight was at the end of a trough which was stepped to deflect incoming fire. In contrast to the tank hulls, the hull side escape hatches were deleted, and the access hatches on the hull front were hinged at the sides.

Starting in June 1940, twenty StuG were built on top of the normal Pz.Kpfw.III hull. These vehicles reatined the hull side escape hatches, front- and rear-hinged access hatches, and the cast cooling air intakes for the steering brakes. The 3.0cm (1.2") hull front armor was reinforced to bring it up to 5.0cm (2.0") thickness. These StuG also featured redesigned crew entry hatches that appeared on Ausf.B.

Ausfuehrung B (Sd.Kfz.142) was powered by a new engine and transmission, and the gunner's hatch and the hatch for his indirect fire periscope were slightly redesigned. The first return roller was also moved toward the drive sprocket as on Pz.Kpfw.III Ausf.H, and the spoked drive sprocket and wider tracks were also introduced during production.

Ausf.F (Sd.Kfz.142/1) was essentially an Ausf.E armed with the longer Stuk 40. The superstructure front was modified for installation of the new weapon, and the ammunition stowage was changed to allow the longer rounds to be stored. Ammunition stowage was modified during production to allow fifty-four 7.5cm rounds to be carried. A ventilator fan was added to the rear roof to clear out gun fumes. Starting in June, additional 3.0cm (1.2") armor plates were welded to the hull and superstructure front. The armor superstructure plates on either side of the main gun were angled more steeply starting in August 1942 so that they met the edge of the roof rather than ending below the roof. The last 31 Ausf.F were armed with the 48-caliber Stuk 40. Vehicles armed with the Stuk 40 were also known as StuG40.

Ausf.F/8 (Sd.Kfz.142/1) incorporated changes also seen on Pz.Kpfw.III Ausf.J, including replacing the split hull maintenance hatches with single-piece hatches and creating tow hooks by extending the hull sides. The split hatches, however, reappeared in November 1942. The additional armor plates were bolted instead of welded to the vehicle starting in October 1942. The engine air intakes were enlarged to improve engine cooling; armored covers were added to the intakes as well. The front fenders on Ausf.F/8 were shorter than previous designs and were no longer hinged. The hatch for the gunner's periscope was made smaller, and a wire screen was added to protect the gun sight. A shield for the loader that was used when firing the machine gun was introduced in early 1943, and schuerzen were retrofitted starting in December of that year.

Ausfuehrung G (Sd.Kfz.142/1) replaced the earlier superstructure with a wider one featuring a commander's cupola. The panniers of previous assault guns were incorporated into the new superstructure, and the rear roof was taller than the old design. A shield for the MG was added in front of the loader's hatch starting in December 1942. Starting in January 1943, the ventilator fan was moved from the roof to the rear wall of the superstructure to further increase room for the commander and loader, and an armored guard was added to the gun sight opening in the roof. From February 1943, the driver's twin periscopes were no longer fitted. Cupolas installed from September 1943 to August 1944 could not rotate due to a ball bearing shortage. During production, the viewing block to the left of the driver was replaced by a plugged pistol port. The front armor was changed to 8.0cm (3.1") thick instead of the laminate construction of earlier vehicles starting in May 1943. The driver's plate, however, remained at 5.0cm (2.0") with additional armor bolted on. A shot deflector was installed around the front of the commander's cupola starting in October 1943. The cast saukopfblende gun mantlet replaced the welded mantlet beginning in November 1943, but the welded mantlets were still being produced up to the end of the war. The external MG mount was changed in April 1944 to one that could be operated by the loader from inside the vehicle, and the naehverteidigunswaffe close-in defense weapon was mounted starting in May 1944. A coaxial machine gun port was added to the welded gun mantlet starting in June 1944, and the cast mantlet in October of that year. Track return rollers were changed to all-steel wheels later in production to conserve rubber. The final drives of the StuG and Pz.Kpfw.III were strengthened in mid-1944 to compensate for the increase in weight of later designs.

From March 1943, some Pz.Kpfw.III returned for overhaul were diverted to StuG production. These vehicles were fitted with the current StuG superstructure on top of the basic tank chassis, and applique armor was bolted to the hull. One hundred forty-two chassis for StuG Ausf.G were completed by M.A.N. as normal Pz.Kpfw.III Ausf.M. These chassis featured tank components like the deep-water fording gear, single-piece hull access hatches, hinged fenders, and welded vice bolted applique armor.