Extension Types in Detail: Single Extension, Full Extension and Over Extension

June 29, 2026

Extension types define in furniture construction how far a drawer can be pulled out of the cabinet carcass. A single extension exposes about 75 % of the drawer depth, whereas a full extension extends to 100 %, offering complete access. An over extension pulls out beyond the front edge of the carcass, providing maximum clearance when reaching in from above.

What Exactly Are Extension Types?

The mechanics of the drawer runners determine the extension travel. This is defined by the number and construction of the moving runner profiles. Simple systems consist of two interlocking profiles, whereas more complex systems are constructed from three or more profiles. The mechanism of a full extension runner operates like a telescope, deploying in multiple stages to its full length. The choice of extension type influences ergonomics, accessibility to the rear of the drawer, and the overall balance of the furniture.

Types and Variants in Detail

Depending on storage requirements, different extension types are used, differing fundamentally in their design.

Single extension: With this variant, the rear quarter of the drawer remains inside the carcass. The travel length is shorter than the nominal length of the runner. This design is frequently found on classic roller drawer runners or simple ball bearing runners.
Full extension: Here, the travel length matches the entire depth of the drawer. The user enjoys unrestricted access to the complete contents. Three-part runner systems are required for this mechanism. In practice, full extension drawer runners are frequently installed, and they can additionally be combined with comfort features such as soft-close drawer runners or handleless mechanisms like push-to-open drawer runners.
Over extension: The drawer pulls out further than its actual depth. The travel length exceeds the nominal length. This is achieved through elongated intermediate profiles and is mainly used for filing drawers or specialised machinery cabinets where vertical top access is strictly required.

Dimensions, Standards and Typical Values

The sizing of runners depends on the carcass depth and the anticipated weight.

Nominal lengths: Runner lengths align with standard grid patterns and typically range from 250 mm - 800 mm. For shallow bathroom furniture, 250 mm drawer runners are ideal. In kitchen and office environments, 400 mm drawer runners and 500 mm drawer runners represent the most common dimensions.
Load capacity: A standard single extension runner usually carries between 20 kg and 25 kg. Full extension runners for residential settings are rated for 30 kg to 45 kg. For workshops or deep pan drawers, the leverage effect demands specialised heavy-duty drawer runners capable of supporting loads from 80 kg to over 120 kg.
Installation width: For side-mounted ball bearing runners, the gap between the drawer side and the internal carcass wall must be calculated precisely. The standard measurement here is generally 12,7 mm (half an inch) per side.

Application & Selection

The decision on which extension type to choose depends on the intended purpose. Single extensions suffice for shallow desk drawers or small miscellaneous items where the rear area is rarely needed. Full extensions are the benchmark for kitchen drawers, ensuring pots and provisions can be lifted out without catching.

When the mechanism must remain concealed, joiners opt for undermount drawer runners. These are mounted beneath the drawer base and frequently feature integrated closing mechanisms directly from the factory. To prevent the drawer front from slamming against the carcass, pairing them with self-closing drawer runners makes sense, pulling the drawer mechanically shut over the final few centimetres.