I would like to see more photos of the area to get a better perspective. It looks a very rough area and a lot of work will be involved, including building substantial steps to get up to the sitting area. Quite frankly, I would do away with grass altogether, it won't be easy to mow, especially if you are limiting it to a small area, and put a timber deck in, which could form a viewing platform for the rest of your garden. From there a simple rockery would take care of the natural slope, leaving generous pockets for plants to spill over. Do you need a play area for children at all? Have you factored this into your garden?

You could go wider but it depends on what type of dining furniture would want to use and how many you will be catering for. Bench seating tucks underneath and when it is being used you aren't so worried about the other space. You could always turn it around the other way [ perpendicular to the house] so the width is less of an issue as 10 metres is a decent length deck. I think even an extra 30cm would make a difference and not doing it may cost more in regret than the materials. You could go wider in the dining area only but it is a bit hard to judge without seeing the space. Find some furniture you like online and map out a plan for yourself - use newspaper for life size modelling. Good luck

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