Pre-Construction Information for London Extensions: What It Is and Why It Matters

**What is pre-construction information?**

Pre-construction information (PCI) is a collection of information about an existing building and site that is relevant to the safety, health, and welfare of those who will design and construct new works on or within it. The concept comes from the Construction (Design and Management) Regulations 2015 (CDM 2015), which is the principal piece of UK legislation governing health and safety in construction.

• For a domestic London extension or loft conversion, pre-construction information typically covers:
• •The existing building: age, construction type, structural system, known hazards (asbestos, lead paint, contaminated materials)
• •The existing services: the location and depth of gas, water, electricity, telecoms, and drainage services on and adjacent to the site
• •The ground conditions: any known ground contamination, previous uses of the site, existing archaeological interests
• •The site environment: adjacent structures, party walls, trees with Tree Preservation Orders (TPOs), overhead power lines
• •Any existing surveys or reports already held by the client: structural surveys, asbestos management surveys, drainage surveys, environmental assessments, planning constraints

**Who prepares the pre-construction information?**

Under CDM 2015, the obligation to prepare and provide PCI rests with the client — though in practice, the client relies on the design team and the principal contractor to help identify what information is needed and to gather it.

*For domestic projects with a principal contractor (most London residential extensions and loft conversions)*:

Where a domestic client appoints a principal contractor for the project, the CDM 2015 regulations impose specific duties on that principal contractor in relation to information. However, the client still has the primary duty to provide the pre-construction information that is already known or can be obtained by them.

• In a design-and-build arrangement (where RCB acts as both design manager/coordinator and principal contractor), RCB's pre-construction team will:
• •Identify the PCI required for the specific project
• •Request or commission the surveys and reports needed to complete the PCI package
• •Compile the PCI package and ensure it is available to all relevant designers and subcontractors before works start
• •Keep the PCI updated as additional information becomes available during the pre-construction phase

*For projects appointing a Principal Designer*:

• Where the project is of sufficient scale or complexity to formally appoint a Principal Designer under CDM 2015, the Principal Designer takes responsibility for the PCI as part of their duties — coordinating the pre-construction phase, managing design risks, and ensuring the PCI is complete before the construction phase begins. The Principal Designer's obligations include ensuring that the PCI:
• •Contains all information known about the site's existing condition that is relevant to design and construction
• •Identifies all significant pre-construction risks and how the design has addressed them
• •Is provided to the principal contractor before the start of the construction phase

**What CDM 2015 actually says about pre-construction information (domestic projects)**:

For a domestic project (a project carried out for a domestic client), CDM 2015 regulations apply in modified form:

• *Schedule 2 of CDM 2015* sets out the content of pre-construction information:
• •A description of the project including its nature, the location of the site, and the client's brief
• •Hazards on or around the site, including: any underground or overhead utility services; the results of any ground investigation or surveys; asbestos surveys
• •Information about the existing structure (where work is in or on an existing building): the structural system; any loading constraints; known presence of hazardous materials
• •Information about the surrounding environment: existing adjacent structures; site access constraints
• •Information from any surveys or reports already held by the client

For a standard London Victorian terrace extension or loft conversion, the most critical elements of the PCI are: 1. The asbestos survey (or confirmation that no notifiable asbestos-containing materials are present) 2. The existing services survey (confirming the location of gas, water, electricity, and drainage services) 3. Any structural survey reports already held 4. The drainage CCTV survey (particularly relevant where the extension footprint may be near existing drainage) 5. Any Tree Preservation Order information (relevant where excavation near trees is planned) 6. The planning decision notice and any pre-commencement planning conditions that affect how the works must be done

**1. Asbestos survey — the most critical PCI element for pre-1980 London properties**:

Asbestos-containing materials (ACMs) were widely used in UK building construction until they were banned in 1999. For London Victorian terraces, Edwardian houses, inter-war semis, and post-war flats built before 2000, the potential for asbestos-containing materials is very real. Common ACMs in London pre-2000 residential properties:

• •Artex ceiling texture (often contains chrysotile asbestos — white asbestos)
• •Pipe lagging and insulation around boilers, pipes, and hot water cylinders (amosite, crocidolite, or chrysotile)
• •Floor tiles and tile adhesive (vinyl floor tiles, particularly thermoplastic tiles from the 1950s–1970s, often contain chrysotile)
• •Roof sheets and soffits (particularly on garages, flat roof extensions, and outbuildings — chrysotile cement sheets)
• •Partition boards in bathrooms, kitchens, and around boilers (Asbestolux, Supalux, asbestos insulation board — AIB)
• •Toilet cisterns and window boxes (particularly in pre-1970 properties)

*Types of asbestos survey*:

• •Management survey (Type 2): A non-destructive survey of all areas, locating and assessing the condition of ACMs that are likely to be disturbed during normal occupation. Recommended for all London pre-2000 residential properties. Surveyor produces a management plan listing all ACMs found.

• •Refurbishment/Demolition (R&D) survey (Type 3): A more intrusive survey required where building works will disturb the fabric of the building — lifting floor tiles, opening ceiling voids, removing partition walls. The R&D survey is the appropriate type for most London extension and loft conversion projects, as the construction works will inevitably disturb existing fabric.

For a London loft conversion or rear extension on a Victorian terrace: an R&D asbestos survey of the loft space (if the existing loft void is accessible) and of any areas that will be disturbed by the works is strongly recommended before design finalisation and before the principal contractor mobilises. Cost: £250–£600 for a standard London residential R&D asbestos survey.

If ACMs are found, they must be: managed in place (if not in the disturbance zone and in good condition); or removed by a licensed asbestos removal contractor before the main building works begin (if in the disturbance zone). The removal cost depends on the type and quantity of ACM — typically £500–£3,000 for a standard residential removal (pipe lagging or small area of Artex); significantly more for larger quantities of AIB.

**2. Existing services survey and utility records**:

Before any excavation or structural works, the position of all existing underground and overhead utility services must be confirmed. In London, the utility services are dense and often poorly mapped:

• •Gas service pipe: typically 600mm below the footway/road surface, but may be shallower in gardens; rising main enters the property at gas meter position
• •Water main and service pipe: water main in the road at 750–900mm depth; service pipe typically enters the property at 750mm depth in the footpath/garden
• •Electricity supply (LV cable): typically 450mm below footway; can be at any depth in a garden
• •Telecoms (BT Openreach, Virgin Media): variable depth; typically 350–600mm below footway; may be at much shallower depths in gardens
• •Drainage (private and public): variable — see drainage surveys guide

• Services can be checked via:
• •LSBUD (Lines Searcher Before U Dig) — the free online utility records service; provides records from most utility companies within a 5m radius of the site
• •Each individual utility's own records request service
• •On-site CAT scan (Cable Avoidance Tool — a hand-held device that detects live cable and pipe signals) — required before any mechanical excavation

**3. Structural surveys and reports**:

• Where the client holds existing structural surveys (e.g., from the original property purchase — the RICS HomeBuyer Report or Building Survey, or a specific structural survey commissioned for a previous project), these should be included in the PCI. The existing structural survey may contain information about:
• •The existing foundation type and depth (important for designing the extension foundation and party wall notices)
• •Known structural defects or previous repairs
• •Soil type and ground conditions
• •Subsidence or tree root movement history

Where no structural survey exists and the property is old (pre-1900 Victorian terrace) or shows visible cracking or movement, a specific structural survey should be commissioned as part of the pre-construction phase before design is finalised.

**4. Tree Preservation Orders (TPOs) and root exclusion zones**:

• London has an exceptionally large number of protected trees — trees protected by a Tree Preservation Order (TPO) made by the London Borough, which prevents cutting, pruning, or affecting the roots or canopy without consent, and trees in Conservation Areas, which have automatic protection. Where any extension excavation (foundations, drainage, paving) falls within the root protection zone (RPZ) of a TPO tree:
• •A tree impact assessment and method statement must be prepared before works start — typically commissioned from an arboriculturist (tree consultant)
• •The excavation method within the RPZ must be specified (typically no mechanical excavation within RPZ; hand dig or air spade only)
• •The foundation design within the RPZ may need to be modified (deeper piles rather than strip foundations to avoid major roots)

The RPZ is calculated as follows: for a tree with a stem (trunk) diameter of D mm measured at 1.5m above ground level, the RPZ radius = 12 × the stem radius (= 12 × D/2 mm). This means a London plane with a 400mm diameter trunk has an RPZ radius of 2.4m.

**Assembling the PCI package — the RCB pre-construction process**:

In RCB's design-and-build pre-construction process, the PCI is assembled as follows:

• *Stage 1 — Client document check*:
• At the first pre-construction meeting, the client is asked to provide all documents they hold about the property:
• •The original planning drawings and any previous planning decisions
• •Any structural surveys, HomeBuyer Reports, or RICS valuations that contain structural observations
• •Any previous contractor quotes, schedules of work, or completion certificates for previous extensions
• •Any asbestos management surveys already commissioned
• •Any title plan or boundary survey information
• •Any existing drainage plans or drainage survey reports
• •Any TPO records (the client can check via their Borough's online planning portal)

• *Stage 2 — Gap identification*:
• RCB's pre-construction team reviews the documents provided and identifies the information gaps — the items that are critical for the project design and programme but are not yet available:
• •No asbestos survey? Commission R&D survey
• •No drainage survey? Commission CCTV drain survey and Thames Water check
• •Unknown ground conditions? Commission trial pit investigation or dynamic probing
• •TPO tree in root protection zone? Commission arboricultural assessment
• •No utility records? Run LSBUD search and arrange on-site CAT scan before excavation

*Stage 3 — Commission the missing surveys*: RCB coordinates the commission of the required surveys and reports, using vetted specialists. The survey reports are received and incorporated into the PCI package. Typical timescale for this stage: 2–4 weeks.

• *Stage 4 — PCI package compiled and distributed*:
• The completed PCI package is compiled and shared with:
• •The project architect (for design review and incorporation into the specification)
• •The structural engineer (for foundation design and structural loading assessment)
• •All subcontractors working on the project (particularly relevant for the groundworker and the plumber — who need to know about existing drainage and services before they start)
• •The Building Control Inspector (the PCI forms part of the information provided at the Building Regulations application stage)

*Stage 5 — PCI updated during the construction phase*: As the construction works begin and additional information is uncovered (e.g., a drainage run discovered during excavation; a structure type revealed during strip-out; additional ACMs found during opening up), the PCI is updated and redistributed. This ongoing information-sharing is part of the principal contractor's CDM duties.

**The practical benefits of good PCI for a London homeowner**:

1. *Fewer surprises on site*: The most common causes of London extension cost overruns are 'unknowns' — information that was not known before the contract was signed and becomes a variation cost when discovered on site. A thorough PCI package does not eliminate unknowns (no survey can find everything before excavation begins), but it significantly reduces the number and cost of unknowns.

2. *Better design*: Architects and structural engineers cannot design accurately without accurate information about the existing building and site. A foundation design based on an assumption about the existing ground conditions may need to be redesigned on site at significant cost and delay. A foundation designed from actual ground investigation data is accurate from day one.

3. *Legal protection*: A client who provides good PCI to their contractor is in a stronger legal position if a dispute arises about unforeseen conditions — because they have demonstrated due diligence. A client who withholds or fails to gather PCI available to them may be held partially liable for costs arising from conditions they could have discovered.

4. *Safer construction*: The fundamental purpose of CDM 2015 and the PCI requirement is safety. Workers are protected when they know what hazards to expect — asbestos that is identified and managed before the plasterer starts is infinitely safer than asbestos discovered as it is being cut through.