Pour Concrete Slab after timber frame garage built?

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It is possible to pour a concrete slab after a timber frame garage has been built, though it may require careful planning and some workarounds to avoid damaging the existing structure. Here's a general outline of how you might approach it:

1) Removing the Floorboards
Since you mentioned the floorboards can be removed easily, this is a good start. Once they’re out, you will have access to the joists and the underlying ground, which is essential for preparing the base for the concrete slab.

2) Reinforcing the Ground
If the ground beneath the paving slabs is stable and compact, you may still need to excavate a little to create a proper base for the concrete. This could involve:
- Removing the paving slabs
- Compacting the soil beneath
- Adding a layer of hardcore or gravel to improve drainage and prevent the slab from cracking over time.

3) Adding a Damp-Proof Membrane (DPM)
Before pouring the concrete, you’ll need to lay a damp-proof membrane (DPM) to prevent moisture from rising through the slab. This is crucial to prevent damage to both the slab and the timber frame in the long run.

4) Pouring the Concrete
You could pour the concrete into the exposed joist areas, but the key will be ensuring the depth and thickness meet local Building Control requirements. Typically, a concrete slab should be at least 100mm thick (though local regulations might vary), and you might also need to reinforce the concrete with steel mesh.

Since the timber frame garage is already in place, you'll want to be careful to prevent the wet concrete from damaging the timber. You could use temporary formwork or barriers to protect the garage's walls during the pour.

5)Potential Height Adjustment
Depending on how high the concrete slab needs to be, you might end up with the new slab sitting higher than the original floor. You may need to adjust the joists or the floor's overall design if you want to ensure the final floor is at the correct height.

6) Consulting with Building Control
Since Building Control is already involved, it would be wise to consult them before proceeding. They may have specific requirements or suggestions on how to remedy the situation without having to take down the entire structure.

Alternative Options:

Screeding or Thin Concrete Over Existing Paving Slabs:

In some cases, you might be able to pour a thinner concrete screed or topping over the existing slabs without removing them, though this might not satisfy Building Control if a full slab is required.

Structural Beams with a Concrete Topping:

Another option could involve adding structural beams (e.g., steel or stronger joists) and pouring a reinforced concrete topping over them. This would allow for a stronger foundation without full excavation.

Conclusion:
Pouring a concrete slab is feasible, but you will need to ensure the ground is properly prepared, the slab is of adequate thickness, and that it doesn’t interfere with the timber structure. Consulting Building Control early in the process will ensure that whatever approach you take will be acceptable.

Unfortunately, I think building control will want the structure taken down . When building a structure of this magnitude, you would need a sufficient strip foundation or potential raft foundation dependant on ground conditions . You would then require a brick damp course so the timbers are not resting on the damp ground.
Over site floor to be installed with 1200 gauge Dpm and concrete slab with steel mesh to maintain long lasting structural integrity .
I would go back to your builder and ask for a refund or ask them to build it properly