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BUILDINGS WITHIN 3M OF A SEWER MAY HAVE TO BE REALIGNED

TECHNICAL & PRACTICE

You just need one officious building inspector handling a scheme and the relatively straightforward process of getting a building built turns into the sort of bureaucratic nightmare that most architects can do without.

Sheila Bull and Jon Corpe bought a fairly derelict property in East Sussex with the intention of improving it to habitable standards. The site location is in a densely populated residential 'old town' area with narrow lanes to the front and rear.

Even though they didn't intend to engage in any significantly contentious work, they still made the usual CON29 DW drainage search enquiries, and the water utilities forwarded a plan marked up with the drains and mains - although devoid of inverts or stipulations on how to deal with these buried services.

Because their proposal for a two-storey dwelling involves demolishing more than half of the existing structure, it falls under the classification of a new-build project. This brings the work under the cover of the amended Building Regulation Approved Document H4: 2000 (which came into effect in April 2002) Building Over Existing Sewers. The regulations determine that any 'construction, extension or underpinning of a building over or within 3m of the centreline of an existing drain, sewer or disposal main shown on the Sewerage Undertaker's sewer records' shall comply with rules governing imposed loads bearing directly upon the sewer and on access requirements to maintain the sewer. There is a main drain running the length of the front lane, which was installed just after the houses were built in the mid-19th century.

Because the lane along which these historic properties are situated is a narrow, winding Victorian alley, the 3m zone of influence extends beyond the curtilage of those properties.

The affected curtilage in this case is not a garden wall, but the front elevation of a row of historic cottages. So any structural work to a building such as this risks falling under the building inspector's insistence that new-build work on the existing property line has to comply with the regulations. The intention is to make sure that no excessive loading is exerted on the buried services and mains, but this reading of the regulations is being imposed regardless of the fact that there has allegedly been no evidence of settlement problems along the sewer.

Obviously, the regulations are there justifiably to maintain the sewer network from reckless damage caused by otherwise unregulated construction. However, in this case the construction proposals are for a tiny timber-framed construction effectively replacing the 150-year-old solid-brick construction that stood there originally. Indeed, the floor area of the original property, which Bull describes as 'no more than a beach hut', is a mere 20m 2 and the proposed building will be constructed off the same footprint. It will impose a total bearing load of 10kN/m 2, which is equivalent to pavement loadings. Therefore, the belief that the new structure might impose point and distributed loads detrimental to the sewer seems unlikely, but sticking to the letter of the regulations imposes severe financial and timescale pressures on any client wishing to repair or rebuild on these plots.

Currently, the engineers for the scheme have drafted proposals for a 450mm-deep foam concrete raft with no bearing within 3m of the sewer, but this has been rejected by the local authority in favour of an expensive piling solution. This, as the client suggests, may be enough to scupper the scheme altogether and is unnecessary with regard to the actual scale of the problem, if indeed, such a problem exists. It also draws the client into expensive party-wall contracts that might otherwise be avoided.

As we go to press, the client and engineer are exploring 'a natural version of a cofferdam sheet caisson' which will provide the necessary stability while minimising any unpredictable sub-structure damage. The client explains: 'This is not possible with the [piling option], as a full shear plane is required under the slab.

In the world of self-build, piles are prohibitively expensive unless there are lots of them, but it's really a matter of appropriateness more than cost.' In this instance, the architect and engineer were arguably unaware of the proximity to the drain, its depth and the fact that water companies - through the offices of the local authority, and hence the Building Control Officer - have executive powers to demand the realignment or, at least, impose design suitability criteria on any foundation within that 3m zone of influence.

It is currently not mandatory for 3m zones to be marked on the deeds - or within the solicitors' searches - and will not be so until 2007. Until such time, it is recommended that detailed site investigations be carried out and that architects be on their guard.

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