MODULAR SAUNAS
The expression "modular sauna" is just another term for a self-build or kit based sauna. It may relate to a materials only kit or a complete prefabricated system where a minimum amount of DIY is required.
Some manufacturers and suppliers like to call their systems "kits" or "self build" units, whilst others prefer to describe them as "modular".
From a buying perspective the terms are interchangeable and they do not indicate any real difference in the system, either from a parts supplied or an assembly perspective.
The important distinction in sauna kit terminology is the difference between a parts or materials only supply, versus a prefabricated supply.
In the case of the former, all of the parts required to build a home sauna are included, but they need to be measured, cut and fitted by the purchaser.
In the case of the latter (i.e. prefab or prefabricated) the system is largely pre-assembled and typically only requires the bolting together of pre-built sections like the walls, floor and roof.
If you are buying a home sauna, be it described as a kit or modular system, find out if it is prefabricated or materials only, then make your decision.