If you intend to plant a tree you should know that the roots will eventually be the size of the tree's height as they grow laterally. That triangle looks rather too close to the house for a tree in my opinion as the roots can interfere with both drains and foundations.

the roots will also do down so do consider the ultimate size of the tree.
welcome to GoY too :o)

Have you built the planter on top of the patio or removed the hardstanding first? If the patio is still under the gravel there will not be enough depth of soil for a tree unless you choose one on a dwarfing rootstock and there also needs to be profision for drainage.. As you have already got the tree could you say what it is please?
Also can you clarify what you mean by planting on a mound and filling in the rest?

Another thought - a tree planted there will soon overhang the boundary -if that fence is a boundary with your neighbour it could cause some ill feeling.
It is important we know what the tree is and whether the depth of soil stops at the level of the patio - you can reply as a comment under the rest of the replies.