Once you have chosen the site for your greenhouse, remove any weeds from the area, as it will be difficult to eliminate persistent weeds once the greenhouse has been built. If you dig the site over, allow the soil to settle for a few weeks before compacting it with a heavy roller prior to the start of construction. Any greenhouse must be built on firm, level ground to prevent the frame from becoming warped or twisted, which can cause the glass panes to crack.

Brick Base

The majority of wooden greenhouses require a brick base with concrete footings to be built before work can begin. Brick wall bases must be constructed precisely (i.e. to the millimetre) and must be absolutely level and square.

1. Mark out the site of the foundations with string lines stretched between wooden pegs.
2. Dig a hole larger than the dimensions of the greenhouse and 13 cm (5 in) deep.
3. Check that the base is level and the sides vertical.
4. Soak the trench with water and allow it to drain. Then add 5 cm (2in) of hardcore in the bottom and tamp it down.
5. Pour in 8 cm (3 in) of concrete, slicing into the mix with a spade to work it in and dispel air bubbles.
6. Compact the concrete by tamping it down firmly with a length of timber, and ensure that it is at or below soil level. Leave the surface rough as a key for the mortar for the course of bricks.
7. Allow the concrete to harden completely for a few days before laying the bricks.
8. The brick base must be laid very precisely according to the measurements provided by the greenhouse manufacturer.

Concrete Foundations

Aluminium greenhouses larger than 2 m wide and 2.5 m long (6 x 8 ft) should be erected on a substantial concrete foundation. It is important to note that the area of your foundation must be larger than the nominal size of the greenhouse; a 6 by 8 foot greenhouse is seldom exactly that measurement; the actual dimensions are usually a few inches bigger (e.g. 6'3" by x 8'5"). Always find out the exact measurements of your greenhouse from your supplier BEFORE you start to construct the foundations.

1. Dig a trench 25 cm (10 in) deep to match the dimensions of the greenhouse.
2. Fill the bottom 15 cm (6 in) of the trench with hardcore.
3. Insert the anchoring bolts.
4. Fill the remaining 10 cm (4 in) of the trench with concrete, making sure that it is level.

Other Bases

Paving slabs may be used instead of concrete foundations, using screws to secure the base of the greenhouse into position. Slabs have the advantage that they will provide natural drainage inside the greenhouse by allowing water to escape through the gaps between them.

Smaller, lightweight aluminium greenhouses may simply be secured using ground anchors; dig a 25 - 45 cm (10 - 18 in) deep hole at each corner, place in the ground anchors, wedging them with rubble. Pour in a concrete mix to secure them firmly.