Thanks for the replies, even your one "happy builder" haha I've got a room i use as an office, it's just to try and keep as much of the noise from the rest of the house out! Thanks again
Id try something simple first like putting a stick on rubber seal around the edges and a brush strip on the bottom. That should reduce a bit of background nuisance noise.
Sound travels through air and is stopped best by solid objects, so you need to have a door that has some decent mass to it and also seal up any air gaps around the door. I've just installed a connecting door between a couple of self catering flats we have so I faced the same question. You need to start with a decent fire door that has some mass to it. If you get a flush fire door you can then add additional layers to bulk it up more if required. In my case I added 5mm of self adhesive Tecsound to one side, overlayered with a sheet of 12mm MDF to give even more mass and a decent surface to paint. With regard to closing air gaps, I have a smoke (brush) seal inset into the sides and top of the frame, plus a smoke seal threshold. Thus, on all 4 edges, the seals brush up against the door and reduce noise transfer around it. In addition, I have self adhesive foam seal strip on the top and side door jambs as a secondary line of defense. Finally, I have a euro lock rather than a traditional mortice, so as to avoid a hole through the door where the key goes in. The door is weighty (so allow 3 or 4 good ball bearing hinges) and closes tightly against all the seals. It's not sound PROOF but it's pretty damn good and (I think) as good as it's possible to get bar bricking up the opening!