Platforms & Changes of Height
Generally, any change of level greater than 300mm should be notified to the Local Planning Officer for approval. Although this may seem rather unnecessary, this rule is important for two primary reason; one – to prevent garden levels changing to allow one neighbour to overlook the other, thereby infringing on their right to privacy, and the second is to recognise the potential effect of raising levels in one garden to the detriment of the next garden in relation to surface ground water moving across a site, whereby one garden becomes flooded by the actions of the site lower down a slope. This regulation is enforced in every area, some more rigorously than others. As with all planning regulations (see Planning Permissions) the onus if upon the home- owner to apply, although the designer/specifier/contractor also has a role to play in ensuring due compliance with the Law.
Measurements are established by existing ground levels, taken from various points around a site. Thirty centimetres from the existing ground level becomes the datum point for design purposes.
The term ‘Platform’ in this article is to introduce the ubiquitous raised patio or terrace, where a patio is constructed at or about the same level as the house doors, often requiring a raised mezzanine or paved platform.
These platforms require retaining walls to be constructed, and the amount of effort, skill and expense that is required for such projects is often underestimated. As shown in the cross- sectional diagram below, not only does the wall require substantial foundations, reinforced as a general rule by the use of 18mm steel rebars, but also hollow concrete blocks filled with concrete with vertical rebars set into the foundations to prevent movement, resulting in a settlement of the sub- base under the paving with obvious results.
Note too, the recommendation of installing a reinforcing mesh into the laying bed, extruded out to sit on top of the retaining wall blocks. This is to prevent settlement within the precincts of the blockwork, whereby the MOT Type 1 settles due to the inability to sufficiently compact the material without causing damage to the walls. If in doubt, or in build-up requiring more than 60mm, it is recommended that a 100mm layer of reinforced concrete is installed under the paving. Although expensive, it is far cheaper than dealing with a collapsing patio.
It is important to note that the ‘new’ paving level, as established by measurement, may still become contentious if the new height overlooks the neighbours. Even if two neighbours agree that they are content to overlook/be overlooked, a future owner could complain, resulting in a dispute unless you have such agreements by both Parties in writing at the time of installation.
A classic example of the effects of settlement under the foundations of a raised platform created by the failure to adequately compact the sub-base. The left-hand side of the image is the height of the retaining wall, and the level shows not only a gap under the straight edge, but the bubble indicates a fall inwards to as much as 100mm across the width of the patio. The costs involved in remediating such problems are very expensive.
Note: The foundation detail shown is recommended for clay ground, using a ‘heel and toe’ configuration which helps to prevent the concrete from ‘tilting’ under pressure. On firmer ground, this is not so essential, although it is always wise to err on the side of caution when designing foundations. Any such retaining wall higher than 600mm should be specified by a Structural Engineer, who will be insured for making their proposals based on calculations.
