Extending over garage.

[Ivan14]

I have a garage and utility room integral to my house, but they are in a side of the house is only single-story whilst the rest of the house is two-story.  I am interested in extending over these rooms to basically 'square-off' my house, and want to have an approximate idea of the cost.

The utility room has cavity wall (block inside, brick-skin outside), the garage only has the brick-skin.  All of the current single-story section has a pitched/tiled roof (sloping on all 3 sides up to the main house).

Inside the extension I plan to have a large bedroom with en-suite, plus maybe knock-through one side of the smallest bedroom in the main house into the new area to change it from a single-bedroom into a double.

I am assuming the following: that I will be able to reuse most of the tiles to extend the tiled roof of the main house over the extension (may need some additional tiles, but not very many); that the existing foundation will support an additional floor (the foundations were part of the original house when it was built so should be solid); that I may need to reinforce the single-layer garage part of the wall with cavity and blocks to support cavity wall on extension above.

I have looked at the resources for extensions, but the single-story guidance assumes the extension is ground floor only and need foundations/etc dug.

Any help or advice would be appreciated.  I am currently guessing it will be around £20k all-in, does this estimate seem high or low?

[HiHo]

How big is it?