Hello,
we are living in an end of terrace 3 bedroom house with double cavity walls of in situ poured concrete of 1950s vintage. There is an existing, very poorly constructed single wall block construction garage set to the rear of the side of the house. This 'garage' is made so that the kitchen door opens into an 'alley' to the rear garden, with a narrow 'cupboard door' into the garage. There is a driveway to the front of the garage consisting of two strips of concrete and a dropped path path the side of the house. There are the usual sort of issues of:
-the sewer downpipe comes down into the 'alley', goes under the existing garage floor and joins an inspection chamber to the rear of the garage in the garden.
-it looks like the gas and electric services come in at the middle of the side of the house.
-the dropped path means that if you build it up to the level of the existing driveway, you would bridge the DPC on the side of the house.

My intentions so far are:

- alter existing driveway to create a slab for the base of a new garage in front of existing one, 150 MM above existing driveway height (having put in strip foundations first). Deal with DPC bridging by leaving a 50MM gap at the side of the house ( I would welcome alternative suggestions).
-build new garage in front of old using Celcon 215MM aircrete blocks and thin joint construction. Place OSB3 or OSB 4 faced 150MM thick SIPs (www.siptec.co.uk)on top of the block walls to create the flat roof. The span of the roof would be 3 M. The SIPs would be 120CM wide and wood splines would be used at joints - there would be no other supporting beams under it - what do you think about this from a structural strength point of view? The underside of the SIPs would be covered with plasterboard to meet fire regs.
- waterproof the roof using roofkrete (www.roofkrete.co.uk) - I would welcome your comments on the lack of a ventilation layer and whether a VCL would be required and where it should be placed.

just out of interest, once the new garage was complete, I would re-use the external side and rear wall of the old garage with the roof construction already mentioned over the top (to minimise waste of demolition) - the result being new garage to the front/side of the house and workshop behind it as a single house extension.

I have tried to add a picture of existing - dont know if it will work.

Thanks for any advice/suggestions

rgds

Graham