Extension for bath waste

[hexdex]

OK, so I'm getting a tad confused with all the different kinds of extensions and pipe sizes etc. I have a standard 38mm extendable P trap to fit under our new bath but despite it being extendable it's still falls short of my bath waste by about 50mm. I know I could solve it by raising the trap and adding some pipe to the waste pipe under the floor, but I would rather add the extension to the waste outlet on the bath so I can keep the P trap under the floor boards.

Can some kind soul point me in the right direction for a solution?

Thank you.

 

[denso13]

This will give you an extra 50mm http://www.screwfix.com/p/mcalpine-...ing-white-40mm-x-50mm/71600?_requestid=396615

 

[hexdex]

That would just about do it thanks!