You'll need a few basic tools such as a spade, hammer and spirit level, and hiring specialist equipment, perhaps a cement mixer, would be a good idea along with cajoling some friends to help share the load and quicken the process.
You won't be surprised to learn that there are several methods to build a concrete base with various recommendations as regards the depth and ratio for the concrete mix. So this is a general guide with instructions that can be adjusted to individual preference.
Choosing a fine day when frost isn't predicted, mark out the proposed position of the summerhouse using string. Then dig out the earth to a depth of about 4ins over an area that's slightly larger. (The depth depends to some extent on whether you want the base and therefore the summerhouse flush with or above the surrounding ground). After removing any roots and detritus, the surface should be level and firm.
Now make a wooden frame (referred to as a form) from four 4” x 2” planks, with the internal measurements a little wider and longer than the summerhouse's dimensions. Nail the planks together, making sure they are square by initially using a metal square and then measuring diagonally from each corner.
Fit the form inside the space you've cleared. When you are sure the form is square and level, secure the sides with several wooden stakes. The form can now be filled with 6:1 ballast and cement with water added to make the concrete workable. Hiring a concrete mixer would be a good idea as mixing by hand is very hard work and will slow your progress.
Use a wheelbarrow to transport the concrete from the mixer and spread with a spade, removing any air pockets by chopping into the concrete. The concrete should be just above the height of the form.
As it's being laid take a piece of timber that is a little wider than the shuttering, rest the plank on the sides and by running along the top of the concrete with a sawing motion, you'll reduce the level and compact it. This is called tamping and results in a smooth finish on which the joists or runners below the summerhouse will rest, so ensuring a stable structure.
For a more belt and braces approach, dig out 8ins of earth, make the appropriate sized timber form and then fill with 4ins of MOT1 sub-base, this should be compacted so that it binds together and forms a smooth surface. You may want to a hire a compactor or 'waker plate' to make this job much easier.
Dampen the surface of the sub-base and then lay the cement on top to a depth of four inches.
Although not essential, a damp proof course can be laid over the sub-base, and for larger summerhouses you might also introduce steel mesh into the concrete to add strength. Then follow the same procedure for tamping the concrete.
In hot weather, you should cover concrete with tarpaulin (without it touching the surface) to prevent the concrete drying out too quickly and cracking. The concrete will start to harden off overnight and we recommend 5-6 days before removing the shuttering and installing the summerhouse.