C O N T E X T 1 3 9 : M A Y 2 0 1 5 33 mortar tolerates some natural seasonal movement as the moisture content of the subsoil varies with temperature and rainfall throughout the year. A rigid concrete structure attached to part of a terrace, such as a new basement or underpinning, creates an immovable ‘hard spot’ which is likely to behave differently to older lime-bonded structures.With natural ground movement, it will be the older, more fragile terrace that will fail and crack before the solid concrete structure does. RBKC’s policy has been to allow a new basement in the garden of a listed building where it can be built at a sufficient distance from the house to allow the house to continue to be supported on its original foundations (which are also of significance) without underpinning.A concession is made to allow a narrow connection from the new basement to the house. The council requires all applications to be accompanied by a construction method statement by a chartered civil or structural engineer. As part of this the engineer must estimate the level of movement that is likely to take place in the host and neighbouring buildings.The draft SPD suggests using the scale published in Document 580 by the Construction Industry Research and Information Association and that normally only ‘category 1’ would be acceptable (‘very slight cracking, up to 1mm’). Most terraces in the borough have no rear access into the back garden from the street, so the excavated spoil has to be brought through the house and out of a window to a skip at the front. Clearly all historic finishes (panelling, plasterwork, doors and other joinery) need to be well protected. Any items that are removed (such as windows) should be carefully stored and reinstated afterwards. Other proposed alterations may affect both the setting of listed buildings and the character of conservation areas. Unwelcome features can include oversized skylights or overly deep lightwells which are uncharacteristic, cause light pollution and may require other works such as glass balustrades and means of escape provision. When gathering evidence for our new policy, aerial photography was found to reveal clearly howmuch grass and planting were being lost to patios and hard surfaces on top of the basements. This was in spite of our old policy requiring the retention of one metre of soil above a basement to allow for a lawn and planting. Our first policy required that 15 per cent of the garden should be left undeveloped to provide areas for water percolation and wildlife. This has been increased to 50 per cent in our new policy, which will also help prevent the sterile and artificial appearance of gardens that was found. Felling or otherwise harming protected trees (either those in a conservation area or with tree preservation orders) is also resisted. For an archaeologist, new basements are rather like small quarries. In large numbers they cause piecemeal but ultimately extensive cumulative damage but of course also the potential to discover previously hidden information. The Greater London Archaeological Advisory Service (GLAAS), alongside colleagues in the City and Southwark, provides advice within the remit of the National Planning Policy Framework. Excavations can reveal information of great interest far and wide. For example, finds at St Martin-in-the-Fields redefined thinking on Roman London. Even small sites can reveal much information, such as at the Adelphi in the Strand, which yielded waterlogged Anglo-Saxon timber and wattle structures. There a rich assemblage of artefacts included animal bone, pottery, leather and the earliest Anglo-Saxon coin discovered in London (7th century) on what was then the river frontage of Londonwic, the international trading emporium described by the Venerable Bede. Sandy Kidd explained how the GLAAS uses the significance of the archaeology as its basis for determining how to manage archaeological impact. Archaeological priority areas (or the equivalent) in local plans identify areas of known or potential interest.As for built heritage, it is important to properly understand the significance of the archaeology and the impact of development before a planning decision is taken. Preservation of important remains in situ should be considered and balanced against the need for development, as proper investigation can be difficult in confined spaces and a ‘watching brief’ may not be sufficient. Once any field investigation has been completed, further research and publication should take place,with important sites always triggering full research. This process was illustrated by a site in CheyneWalk, where a 17th-century basement was discovered within the grounds of an earlier house owned by Sir Thomas More. With planning policy and guidance, basements are better controlled and less harmful to heritage assets, and nuisance during construction can be minimised. With adopted guidance and a consistent approach, RKBC is sending a clear message about how we deal with basements, providing residents in the borough with the transparency and certainty that they desire. Justine Page is senior conservation and design officer at the Royal Borough of Kensington and Chelsea. Further reading www.rbkc.gov.uk/ planningandbuildingcontrol/ planningpolicy/ developmentplan/ corestrategypdfs.aspx RBKC Planning Policy CL 7: ‘Basements’, January 2015 ‘Basements’, supplementary planning document (draft), February 2015 ‘Basements in Gardens of Listed Buildings’, Alan Baxter Associates, February 2014 ‘Basements Report’, Alan Baxter Associates, March 2013 A diagram by Alan Baxter Associates showing the cause and effects of differential movement Cracking on a facade
RkJQdWJsaXNoZXIy MjgyMjA=