I need to fit 2 x double sockets in a floor in my wife's studio. they will be under her desk. the problem I have is that the joists are 300mm centres (and the posi-joists are 147mm wide so I have a 153mm gap) and so there isn't enough space for the normal floor box type fixture. 

 

so can I simply use standard metal back boxes fixed in place and then put something like this on top?

 

https://www.tlc-direct.co.uk/Products/FP9UBCW.html?source=adwords&ad_position=&ad_id=&placement=&kw=&network=x&matchtype=&ad_type=pla&product_id=FP9UBCW&product_partition_id=&campaign=shopping&version=finalurl_v3&gad_source=1&gbraid=0AAAAAD-6GhmCN-K7_n0EaBtlr027vsR5-&gclid=EAIaIQobChMI-d-G8JSdiQMVlZlQBh3iWylpEAQYBCABEgKie_D_BwE

 

wondering if there are any regulations against that.

Reckon. "Normal floor box" as in office-style?

45 minutes ago, Alan Ambrose said:

Reckon. "Normal floor box" as in office-style?

yeah. this is what I would consider a "normal" floor box. https://www.tlc-direct.co.uk/Products/UVCSM3N.html

 

even the 1 compartment floor boxes are too wide for me. https://www.cef.co.uk/catalogue/products/1662319-218x150mm-1-compartment-floor-box 

Just a standard 35mm 2 gang metal box will be fine

I’ve got a standard metal back box under my wife’s sewing table, takes a two gang face plate with cover flap thing. 
 

Thanks all.