Question: I've purchased a dishwasher supposedly with exact same dimensions as the previous however swapping out hasn't been as straightforward as I imagined. The issue is a waste pipe, ever so slightly in the way and preventing the dishwasher from a flush fitting. There's about 25mm in it. The connector pipe appears to have an unnecessary T fitting (see pic). I was thinking about swapping out the connector and have 2 questions:

1) Is that T unnecessary? Or is it some type of non-return valve

2) If it's solvent welded I've read that I'll have to saw the pipe off. If so do I cut, insert a new length of pipe and then solvent weld with 2 40mm Straight Coupler's?

Thanks! 

![Waste pipe connector](https://i.stack.imgur.com/3bWnY.jpg)

Answer: 
That kind of T-connection low down suggests that it joins the wastes from two P-traps.

It is probably more usual for dishwashers to connect above the P-trap of an adjacent sink.

![enter image description here](https://i.stack.imgur.com/UxhWD.jpg)  

An "appliance P-trap", the tapered nozzle on the left is to connect the outlet hose from an appliance such as a dishwasher.

You can also get a "standpipe appliance trap"

![enter image description here](https://i.stack.imgur.com/pZXJb.jpg)  

I've not used this type but your appliance hose would be hooked into the top.

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If you can remove one of the outlets that your T-joint combines then you might be able to plan a new layout for the waste pipes that provides more space for your appliance.

It isn't clear to me from your photo, exactly what the problem is and what exactly feed the two inputs to that T-join.

In principle you can cut through the pipes and push on a new solvent-weld (or push-fit or compression fitting) joint. But you need enough pipe remaining after the cut to fit into the coupling. The more times you cut and remake these solvent-weld joints the less and less pipe you have to work with. 

As you suggest, if there is nothing using the vertical waste pipe above the T, you could cut out the T and cut back the existing pipe far enough to fit a new section of the same type of ABS pipe with two straight couplings.
There is not enough information for a complete answer, however, if there is no known drain feeding into the top of the 'T', then it is likely that the T is functioning as the vent for that portion of the drain system. 

It doesn't look like it was designed/installed properly, however, removing it would likely cause trouble. Most things are done for a specific reason - items like this add time and complexity to a project and are not typically added without a good reason (typically).

The issue with your dishwasher is probably because the new unit is more square-shaped on the back compared to the old unit - it fits tighter into the corner than the old and thus, impacting this pipe? Or am I miss reading your photo?

Your proper long-term fix would include pulling the cabinets out entirely and fix the plumbing to both not interfere with the new appliance as well as properly drain and vent the sinks and appliances as needed to meet code requirements. That is not likely to be a small task - ask a plumber for a quote and go from there.