Building Control for London Extensions and Conversions: Full and Building Notice Routes Explained

**Why Building Control is required**:

Building Control is separate from planning permission. Planning permission addresses the appearance, scale, and impact of a development — whether it is acceptable in planning policy terms. Building Control addresses whether the construction itself meets minimum technical standards for safety and performance. Both may be required for an extension — but they are separate applications, separate approvals, and separate inspections.

For some works, only Building Control is needed (not planning permission) — for example, an extension within permitted development limits; a loft conversion that does not change the external appearance of the house; most internal structural alterations.

For some works, only planning permission is needed (not Building Control) — for example, a boundary fence under 2m; a flagpole; a decorative garden structure with no foundation.

For most London extensions and loft conversions, BOTH are needed — and they run on parallel but independent tracks.

**The two Building Control approval routes**:

*Route 1: Full Plans Application*

A Full Plans application is a formal submission of construction drawings and specifications to the Building Control body (BCB) for technical approval before construction starts. The BCB checks the drawings for compliance with all relevant Parts of the Building Regulations (see below) and either approves the drawings, approves them with conditions, or requests amendments.

• *Advantages of Full Plans*:
• •Drawings are checked and formally approved before construction begins — provides certainty that the design is compliant before money is spent on construction
• •Any Building Regulations issues are identified at the design stage (where they are cheap to fix) rather than on site (where they are expensive)
• •A formal decision notice (approval or rejection) is issued — provides legal certainty
• •Useful for complex projects (basements, loft conversions, structural alterations) where Building Regulations compliance is non-trivial
• •Lenders (mortgage companies) and solicitors typically want to see a Full Plans approval or completion certificate for extensions — a Full Plans application provides this assurance

• *Disadvantages of Full Plans*:
• •Requires detailed construction drawings and specifications before the application — which takes time and money to prepare
• •The BCB takes up to 5 weeks to check and issue a decision on Full Plans
• •Additional cost compared to a Building Notice (though the difference is typically small)

*Route 2: Building Notice*

A Building Notice is a simpler notification to the BCB that building work is about to start. No drawings are submitted — the BCB is simply notified, and inspection is carried out on site during construction. The BCB inspector visits at key stages and checks compliance with Building Regulations on site.

• *Advantages of Building Notice*:
• •Quicker to start — work can begin 48 hours after the notice is submitted
• •No detailed design drawings required before starting (though the contractor should have working drawings)
• •Lower upfront cost (no detailed drawings to produce for submission)
• •Suitable for straightforward, well-understood projects where the contractor is confident in compliance

• *Disadvantages of Building Notice*:
• •No formal design check before construction — compliance issues only found on site (expensive to fix)
• •No formal approval notice issued — only a completion certificate at the end; solicitors and lenders may want more formal documentation
• •BCB inspectors may require additional information or remediation if they find non-compliance on site
• •NOT available for some project types — Building Notice cannot be used for: commercial buildings; larger residential buildings (above 18m or with complex fire safety requirements); HMO conversions in some interpretations; any project involving a build-over of a public sewer (Thames Water build-over agreement requires a Full Plans submission in most cases)

**Which route to choose for a London extension?**

| Project type | Recommended route | Notes | |---|---| | Standard rear extension (1-storey) | Either — Building Notice is adequate | Full Plans gives more certainty | | Loft conversion | Full Plans recommended | Staircase, fire safety, structural complexity | | Basement conversion | Full Plans required (strongly) | Complex waterproofing, structural, drainage | | HMO conversion | Full Plans required | Complex fire safety requirements | | Extension with public sewer build-over | Full Plans required | Thames Water requires drawings | | Structural alterations only (no extension) | Building Notice adequate for most | SE drawings still needed | | Listed building | Full Plans required | LBC also needed separately |

**Local Authority Building Control (LABC) vs Approved Inspectors (AIs)**:

Since 1984, Building Control work in England has been open to private sector Approved Inspectors (AIs) as an alternative to Local Authority Building Control (LABC). Both are legally authorised to carry out Building Control functions:

• *Local Authority Building Control (LABC)*:
• •Part of the local authority (London Borough) — the same organisation that handles planning applications
• •All London boroughs have their own LABC service
• •Well-known, stable, can be referenced in future property searches (local land charges)
• •Fees are set by the local authority — typically comparable to AIs
• •Turnaround time for plan checking can be slower in some boroughs

• *Approved Inspectors (AIs)*:
• •Private sector companies registered with the Building Safety Regulator to carry out Building Control functions
• •Well-known examples: NHBC Building Control; Bureau Veritas; Assent Building Control; Stroma; Regional Building Control
• •Must be registered on the Registered Building Inspectors (RBI) register maintained by the Building Safety Regulator
• •Typically quicker turnaround on plan checking
• •Issue a Final Certificate (equivalent to local authority completion certificate) at the end of the project
• •For residential work: AIs must notify the LABC of their appointment — the LABC has oversight

*Post-Grenfell changes (Building Safety Act 2022)*: The Building Safety Act 2022 introduced significant changes to Building Control, particularly for Higher Risk Buildings (HRBs — residential buildings above 18m or 7 storeys). For standard London residential extensions and conversions (below 18m), the most significant change is the introduction of the Registered Building Inspectors (RBI) register — all building inspectors must now be registered. For HRBs, the Building Safety Regulator (BSR) is now the BCB directly — not LABC or AI.

**The Building Control inspection stages for a London extension or loft conversion**:

For a Full Plans or Building Notice application, the Building Control inspector visits the site at specified stages to check compliance before the work is covered up. The standard inspection stages for a rear extension are:

*Stage 1 — Commencement notification*: The contractor must notify the BCB at least 2 days before work commences. This is a statutory notice — failure to notify can result in the BCB being unable to inspect at the correct stages.

*Stage 2 — Excavations/foundations*: The inspector visits when excavations are complete and before concrete is poured. At this stage they check: foundation depth and type; the ground conditions at the base of the excavations; width and depth of strip footings; pile locations and depths (for piled foundations); any drainage runs that cross the foundation area.

*Stage 3 — Damp proof course (DPC) level*: The inspector visits when the external masonry walls have been built to DPC level (typically two courses above finished external ground level) to check: DPC is correctly installed; cavity trays are in position; DPC is tied to the floor DPM.

*Stage 4 — Oversite (floor slab)*: The inspector visits before the floor slab is poured to check: sub-base compaction; DPM continuity; insulation thickness and type; reinforcement specification and cover.

*Stage 5 — Frame/structural elements*: For extensions with structural steel (beams, columns), the inspector may visit when the steelwork is erected to check: beam sizes match the structural engineer's specification; connections are as detailed; padstones are provided under beam bearing points.

*Stage 6 — Roof structure*: The inspector visits when the roof structure is complete but before roofing felt, battens, or membrane are laid. For a flat roof extension: checks rafter or joist sizes, insulation configuration; for a dormer: checks dormer structure, cheek construction.

*Stage 7 — Drainage*: The inspector witnesses a drainage test (pressure test or water test) of the new drainage runs before they are backfilled. For an extension connection to the existing drainage, the underground runs are checked for: correct gradients; access at changes of direction; rodding eyes.

*Stage 8 — Completion/final inspection*: The final inspection, when all works are complete, checks: all Part L (thermal) requirements (insulation, glazing specifications — Building Control may request product data sheets for insulation and glazing); Part F ventilation (extract fans installed, trickle ventilators in windows — may request commissioning certificate for MEV/MVHR); Part B fire safety (fire doors installed, smoke alarms interconnected); Part M level threshold; Part P electrical installation (NICEIC/NAPIT electrical installation certificate required for new circuits).

**Which Parts of the Building Regulations apply to a typical London rear extension**:

The Building Regulations are divided into Parts (A through S as of 2022). The Parts relevant to a typical rear extension:

| Part | Subject | Key requirement for extensions | |---|---| | Part A | Structure | Structural adequacy of new walls, roof, foundations, beams; SE calculations required | | Part B | Fire safety | Escape routes maintained; smoke alarms interconnected; fire-rated construction at boundary | | Part C | Site preparation and moisture | DPM in floor; cavity tray DPC in walls; external wall moisture resistance | | Part E | Sound | Walls and floors at separating walls (shared walls in terraces/semis) | | Part F | Ventilation | Extract fans in kitchen/bathroom; trickle ventilators in windows | | Part H | Drainage | Surface water and foul drainage; build-over of sewers | | Part J | Combustion | If gas appliances added to the extension | | Part K | Protection from falling | Guarding to stairs, balconies, level changes | | Part L | Energy efficiency | Thermal performance of new walls, floor, roof, windows | | Part M | Accessibility | Level thresholds at external doors; minimum door widths | | Part O | Overheating | Passive cooling measures for new habitable rooms (2022 revision) | | Part P | Electrical safety | Notification of new electrical circuits; NICEIC/NAPIT certificate | | Part Q | Security | Windows and doors in new dwellings to meet PAS 24 | | Part S | Infrastructure for charging | EV charging provisions (new dwellings and major renovations) |

**Building Control fees for London extensions (2025 — illustrative)**:

Building Control fees vary by BCB (local authority vs approved inspector) and by project value. Typical fees for LABC applications in London:

| Project | Typical total LABC fee | |---|---| | Single-storey rear extension (<10m²) | £400–£600 | | Single-storey rear extension (10–30m²) | £600–£1,000 | | Single-storey rear extension (30–60m²) | £900–£1,400 | | Loft conversion (standard) | £700–£1,200 | | Loft conversion (large or complex) | £1,000–£1,800 | | Basement conversion | £1,200–£2,500 | | HMO conversion (3–5 rooms) | £1,000–£2,000 |

Fees above are indicative — actual fees depend on the specific local authority and project. Approved Inspector fees are broadly comparable.

**Getting the completion certificate**:

• The completion certificate (from LABC) or Final Certificate (from an AI) is issued after the final inspection confirms the works comply with Building Regulations. This document is critically important:
• •It is requested by solicitors during property conveyancing searches — without a completion certificate for any extension or conversion, a property sale can be delayed or the buyer's solicitor may require an indemnity insurance policy
• •It confirms the works were carried out to the required standard — important for property valuation and mortgage purposes
• •It is required by insurance companies for extensions

*How to ensure the completion certificate is issued promptly*: 1. Book the final inspection when the works are genuinely complete — premature final inspections where work is still outstanding lead to delayed certificates 2. Have the following ready for the final inspection: electrical installation certificate (Part P) from the electrician; gas safe certificate from the plumber (if gas work done); drainage test certificate; commissioning certificate for any MEV/MVHR system (Part F); product data sheets for insulation, glazing, and rooflights (Part L U-values) 3. Chase the BCB for the certificate if not received within 4 weeks of the final inspection — some local authorities are slow to issue certificates once the inspection has passed

**Regularisation — what happens when extension work was done without Building Control?**

• If a previous extension or conversion was built without Building Control approval (this is more common than most homeowners expect — many 1980s and 1990s extensions in London were built without formal Building Control), a Regularisation application can be made to the LABC (not to an AI — regularisation is LABC-only) to retrospectively obtain approval. The process:
• •A regularisation fee is paid (typically 150% of the normal application fee)
• •The LABC inspects the existing works — they may require some opening-up (exposing foundations, floor build-up, insulation) to confirm compliance
• •If the works are found to be compliant (or can be made compliant with minor remediation), a regularisation certificate is issued
• •If the works are found to be non-compliant and cannot reasonably be remediated, the regularisation certificate is not issued — and this must be disclosed in property searches

Professional indemnity insurance (available from specialist legal insurers) can be used as an alternative to regularisation in some conveyancing scenarios — the insurance covers the buyer against the risk of enforcement action.