This project will only apply to your conservatory if it is not governed by building regulations. The construction of a conservatory, for example, which involves the conservatory becoming part of the existing building by removing external doors, or making an opening into the conservatory with no doors on, means the conservatory will be governed by building regulations. This is because it now becomes subject to insulation regulations requiring it to retain the required amount of heat.
Most Double Glazing Companies will help you with your needs and can explain requirements to you if you are going to fit one of their conservatories on a DIY base.
The building regulations will very likely stipulate, in these situations, that the foundations be taken down to 1 m or to local authority approval. The local authority approval guidelines (Building regulation requirements for foundations under table 12) are given in our foundations project.
Just because, in other cases, your conservatory construction is not governed by building regulations does not mean its ok to lay a little concrete and get on with it. For your conservatory to stay upright for any length of time it makes sense to follow the guidelines as they are there for your benefit, not the councils. The ground you lay your foundations on should be firm, not wet and spongy. Dig down to what you believe is solid ground. Cut a square peg, 50mm x 50mm and bang it into the bottom of the trench with a lump or club hammer. You should not be able to drive the peg in any more than 150mm (6inches) without difficulty. When you concrete the mix should be 6 of mixed aggregate (ballast) and 1 cement. See our mixing concrete project for help with this. Measure your foundation trench, it should be wide enough to accommodate whatever thickness of wall you have in the centre, with a 100mm overhang either side.
For example; If you have an internal skin of blockwork of 100mm, plus a 50mm cavity, plus an external skin of 100mm brickwork this is a wall thickness of 250mm. Add 100mm either side of this and the trench width should be 450mm. Again this calculation does not supersede any given with building regulation requirements.