Thinking of moving a 20 year old japaneese maple?

a 5" caliper tree is big to move but doable. You'll want to dig a rootball about 8-10" diameter per inch of trunk caliper. The depth is less important, maybe go two feet.

The ideal way to do this is to "root prune" i.e. slice the roots with a spade around the perimeter of where you'll make the rootball, about 6 months before moving, then dig and transplant during the dormant season. I'm guessing you didn't think that far ahead - but it's not the end of the world. The thing I would do is avoid when it's actively growing - I'm in the Northeast, so I would try to do it in the spring, or if not, wait until summer, when the new growth has hardened off, avoiding, say, June, when it's really pushing new growth.

Water the tree well a few days ahead, so it's fully hydrated, and so the soil will stay together, then dig it, burlap it, move it, and plant it, watering it again after it's moved. Some people swear by products like Bioplex, Superthrive, or Roots. I have limited experience with them so I can't recommend them for sure, but they won't hurt.

There's no need to trim the tree unless you want to do so for other reasons, i.e. pruning branches for aesthetics. Tying up the branches while you dig it will help get them out of the way and reduce breakage.

A tree that bid is going to be hard to move either on you own or with professional help. The root ball is probably rather large and will be hard to get whole, agree with above person to call a tree mover and get a quote. Could build a unique deck and build it around the tree??, have seen this done before. Here are some ideas http://www.houzz.com/deck-around-tree

As well as considering the previous answer, I would suggest seeking the advice of a local professional gardener. There are many factors to be taken into account, that relate to relocating an established shrub/tree.

1. It's current position and the proposed new one.

2. The time of year.

3. The soil type and depth of topsoil.

4. The layout of stem and branch structure.

5. It's relative value to you.

6. The time period available for moving.

7. The aftercare required.

Alternatively there is a cheap and cheerful option.

If your patio contractors are using a digger, you could always get them to dig it out and replant it for you. The rootball needs to be as wide and as deep as the machine will move. If the maple is in leaf, slowly give it several buckets of water 24hrs before moving. It will also require regular watering for next 18months.

Regards, Ian.

For a tree that established and that valuable I would not trim it. I'd call a professional tree mover and let them do it. They have a machine that will dig out the root ball such that it will likely do minimal harm to the tree. They can give you a cost estimate first so you can decide if you can afford it (but I sure would do it).