tiling over different surfaces

First ever post, so be kind please!

I'm renovating my fireplace - a wood burner is going to be installed.
Ideally the hearth will be tiled.
There is an old concrete hearthstone, which will be a good base.
However I would like to tile beyond the concrete, over wooden floor boards by about 15 cm.
The concrete is approximately the same level as the timber, but i'm worried that there will be issues spanning the two surfaces due to expansion/contraction.

Any thoughts, ideas, advice?

Thanks.

You might use a sheet of 6mm or 12mm backer board - cut it to fit your dimensions but allow an expansion gap of 10mm. You could "glue" it down with the tile adhesive.

It will, however, make the tile proud and give an exposed backer board edge but if you are laying carpet or laminate, for instance, then the proud edge wont matter.

Thanks good suggestion, I did just that. The board is indeed hidden by the carpet.