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What you're describing is called furring in the trade. By putting the furring onto the existing floorboards rather than directly onto the studs you could be creating problems for yourself down the line.
When doing furring you need to ensure that the centre of the furring piece is lined up with the centre of the joist. While it is possible to do this blind it is much easier if you can actually see the joist that you're planting it onto. If the centres don't line up then the pressure from the floor above won't be transmitted to the joist and the floor boards will be taking more strain than they should.
The floorboards were never meant to be sandwiched in between two things in this way and there is a chance that over time the fibres of the wood between the joists and the furring could compress. If this happens the floor will start to sink a bit, HSE defines a trip hazard as any change in floor height greater than the thickness of a 50p piece - worth bearing in mind.
If damp gets into the floorboards it will be much harder for them to dry and they will rot. Sorting that out will be an absolute nightmare.
I can see the appeal in not lifting the original boards but I personally don't hunk it's worth it and would never recommend it to a client. Plus last time I checked reclamation yards paid money for old floorboards (I last checked a while ago but it's worth asking).
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Bobbo
Hey all, I am replacing all the floorboards in the house (its tiny) and one room has a 60mm step down which I would like to bring up to level.
The existing floor is pine timber board over joists with subfloor. My idea was to put 40mm timber strips inline with existing joists (new floorboards are 20mm) but leave the pine floorboards in place, then board over the top and put some 40mm insulation board in between the new "fake" 40mm joists.
Is this a really silly idea...