Yes, hair extensions can be applied to short hair, but not to every starting base. The deciding factor is whether the natural hair can conceal the attachments and blend with the new lengths.
What is the main limitation?
There is no universal minimum length that applies to every head. In many cases, several centimetres are needed to provide enough coverage over the attachment points, especially at the top, sides and nape.
A very short pixie may not provide sufficient coverage. A bob, however, can be lengthened with excellent results if the density allows the attachments to be concealed and the cut is blended correctly.
Why is blending so important?
On a short haircut, the most obvious risk is a visible “step” between the natural hair and the extensions. Avoiding it requires correct distribution, an appropriate quantity and a final cut that connects the natural base to the new lengths.
On short hair, application is only half the work. The other half is blending.
Which methods can work?
Keratin bonds, micro-rings, tape-ins and wefts can all be used depending on density, coverage and the desired result. Keratin bonds allow very precise strand-by-strand distribution; wefts and band systems can build volume quickly through central areas when there is sufficient coverage.
To compare the methods, read Human hair extensions: which method should you choose?.