First you need to think about the site for the proposed raised bed, for example will it be next to a building or a wall? If so and you want to grow some plants that have a high water requirement then because the building or wall will act as a barrier to getting rain to fall on the bed you may have to water more often. This is due to the bed being in the ‘rain shadow’ of the wall etc. If you want to grow alpines then this could be an ideal site. Also on what surface do I place the raised bed? If it is going on soil or grass then you need not worry about drainage from the bed. If it is going to be placed on a hard surface and the hard surface is already laid then you will need to think about adding some drainage outlet to allow excess water to escape from the bottom of the bed.
The next thing to consider is the construction, which materials should be used and how permanent do you want it to be. If the raised bed is to be permanent then it could be made from brick with a coping stone on top to act as a seat, this is the most expensive type of construction but also is the most durable. For example wood would need to be replaced after a period of years, how often would depend on the wood type and any treatment that has been applied. Which wood should I use? A naturally resistant wood like redwood or cedar is ideal but pricey and so most people use softwood like pine and treat it with preservative. If using the raised bed for vegetable production then the old treatment of chromated copper arsenate (CCA), a compound using arsenic as its primary rot protectant should be avoided as it could contaminate the soil nearest to it. The last thing to decide before construction begins is the height, width and length. The height will depend on what you wish to grow and how far you want to bend down etc. The lowest height for plants to grow is 6 inches (152mm) but I would suggest for most plants a depth of a foot is sufficient. But for the following uses the guide line height would be for standing about 39 inches (1 m), for sitting 29 ins (760mm) and for wheelchair use 24 ins (615 mm). These are suggested heights and you may need to adjust them for your particular requirements. As to width I usually suggest that you extend your right arm and measure form armpit to wrist and then double that measurement less 1 inch. This means that you can stand or kneel at the side of the bed and then still reach the middle without walking or standing on the soil, the added benefit is that you need never dig the raised bed as it will not become compacted. As to the actual building of the raised bed the simplest construction is to lay the planks on their side with the corners just touching. Next you raise them upright to stand on their edge and then hammer in supports to keep them in position. The trick when raising the boards is to make sure you do not pick them up and thus lose the contact with the other boards. If you use sleepers then place them in position and again hammer in supports to keep them in the correct place. If you come across old creosoted railway sleepers then avoid using them as the seepage from the old wood will cause problems both for plants and your clothes.
Soil to use
We now have the beds built to the height and width you want and so it is time to fill the empty space. But before that is done you need to consider if you are going to line the bed if it is built of wood. If the wood is treated then it will probably last for 10 years plus and should therefore not need to be lined. If the wood is untreated then lining the side with landscape fabric will prolong the useful life of the timber. If you use plastic sheet then this could retain too much water and discourage beneficial insects and worms as it does not allow the movement of excess water through the sheet. Having lined or not the bed you need to fork over the bottom of the enclosed area to stop soil compaction and allow free drainage. If the bed is standing on grass then you just need to pierce the grass surface with a fork at 6 inch (152mm) intervals across the area. If the site is on hard standing then having added drainage holes at the bottom of the wooden sides this is all that is needed.
Now add the soil, here we have a choice, if you are going to grow small plants with a relatively shallow root system then you can use the propriety potting or basket compost mixes we see at the garden centres. If you are going to grow plants or even small shrubs and trees in the raised bed then I would suggest you use a mix of top soil, garden compost and potting or basket compost. If the bed is quite deep (at least two planks or sleepers deep) then I would fill the bottom of the bed with sieved top soil for half the depth and then use the following mix for the top half. Mix 60% topsoil with 30% garden compost or other organic matter and 10% propriety potting or basket compost. The ratios by volume is 6:3:1, so 6 buckets of topsoil, 3 of garden compost and 1 of potting compost. The bed should be filled to the very top so that the soil is level with the top of the sides. This is to allow for settling over the next two weeks and thus it will be just below the level of the sides once this has happened.
Once the settling of the soil has stopped after about two weeks then you are ready for planting. If you have made your raised bed during the winter then wait for spring and the soil to warm up before planting or sowing.