Advice Appreciated for Building a Garage with an Adjoining Outside Toilet

mcac

I'm hoping for any advice from my trusted Money Saving Experts on my building/planning questions.

First just to give a little preliminary info...
We live in a semi-detached property on a corner plot with a fairly sizeable garden in a council area. We bought this property from the LA fifteen years ago.
We'd like to build a single detached garage in our garden with an attached (small) toilet/sink & shower room.
The garage is for my car (not to convert at any point to be 'habitable').
We have only one bathroom in the house, and I miss having the original outside toilet which was knocked down to make way for a kitchen extension some years ago. I would like to add a sink/shower so my husband can get grubby in the garden without then bringing said grubbiness through the house & upstairs to our bathroom. And the convenience of an outside loo would be great.

I have trawled the interactive Planning Portal and Permitted development rules and understand the actual size of the garage and distance from our neighbouring properties & boundaries won't be a problem. However, there are a few questions I just can't find answers for:

1.) Will we need planning permission? If so, what are the chances of us being granted planning permission? And would we be applying for planning for both the garage and shower room or just the shower room?

2.) What would the issues be with drainage of sink/shower/loo?

3.) Is there anything specific that I need to be aware of or avoid?

Any additional information or advice would be much appreciated.
Many thanks in advance.