We are looking to replacing the existing washing machine, and getting a dishwasher. does extra plumbing need to be done?

No extra plumbing is required. Bill Pipe Dreams Plumbing Services Bournemouth

As you are keeping a washing machine and adding a dishwasher then yes a little bit of plumbing as the new dishwater will need a cold water feed and a waiste outlet but both are easy to do as long as the sighting of the the new appliance is somewhere near your kitchen sink or washing machine. If there is easy accesss to waiste and water supply shouldn't take long and if you have enough overhang on the worktop I would put an end panel on instead of a pole as this will hide the floor that was under the old unit if it was not finished as the rest of the floor. In all this job should not take mor than 2 to 3 hours depending on as stated before your water and waiste. There will of course be some plumbing bits but only a washer tap and maybe an outlet attachment for your sink waiste to allow for waiste water. As for price that depends on the hourly rate of the trades man.

You will need a cold supply to the new dishwasher and a way to get rid of waste water you can buy a 2 way splitter for the supply from most DIY places and a dual spigot connection for the waste Basically turn 1 supply and 1 waste into 2 for around £10/15 Cost to remove cupboard approx £40/60 depending on where you live