Sound Insulation in London Conversions and Extensions: Part E Requirements, Robust Details, and Acoustic Testing

**When does Building Regulations Part E apply?**

Approved Document E applies to: 1. **New-build dwellings** (new houses; new flats; new apartments) — any new wall or floor that separates a new dwelling from another new dwelling or from a non-domestic space 2. **Conversions creating new dwellings** — a house converted to two or more flats; a large house converted to an HMO; a commercial building converted to residential flats — any new separating wall or floor created by the conversion 3. **New extensions that create a new separating floor or wall** — for example, building a new storey above an existing flat that creates a new floor between two dwellings

• *When Part E does NOT apply*:
• •Internal walls and floors within a single dwelling — the wall between two bedrooms in the same house; the floor between the ground floor and first floor of a single-occupancy house
• •Existing separating walls and floors in existing buildings where no material alteration is being made (no requirement to retrofit existing separating walls/floors to meet Part E in a refurbishment, unless the separating element is being materially altered or a new dwelling is being created)
• •Party walls between two existing adjacent dwellings undergoing renovation on one side only — the existing party wall continues in its existing condition; there is no obligation to upgrade it unless it is being materially altered

**Part E performance standards — the required sound levels**:

Part E sets two types of sound insulation standard:

*1. Airborne sound insulation (DnTw)*:

Airborne sound transmission is sound produced by voices; music; television; domestic appliances — sound waves transmitted through the air that then cause the separating element (wall or floor) to vibrate, re-radiating sound on the other side.

DnTw (Weighted Standardised Level Difference) is the UK measurement standard for airborne sound insulation between dwellings. The higher the DnTw value (in decibels, dB), the better the sound insulation — more dB means more sound is blocked.

*Part E minimum DnTw requirements (from pre-completion testing)*:

| Separating element | Minimum DnTw (dB) | Notes | |---|---|---| | Separating wall (between two dwellings) | ≥ 45 dB | Any wall separating two dwellings — including a converted house where an internal wall becomes a separating wall after the flat conversion | | Separating floor (between two flats/dwellings) | ≥ 45 dB | The floor between a lower flat and an upper flat — applicable to new-build and conversions |

*2. Impact sound insulation (L'nTw)*:

Impact sound is sound generated by physical impact on a floor — footsteps; dropped objects; chair legs scraping — which transmits directly through the floor structure as a structural vibration and re-radiates as sound in the room below.

L'nTw (Weighted Standardised Impact Sound Pressure Level) measures impact sound transmission. LOWER L'nTw is better — less impact sound transmitted means better insulation:

*Part E minimum L'nTw requirement (separating floors)*:

| Separating element | Maximum L'nTw (dB) | Notes | |---|---|---| | Separating floor (between two flats/dwellings) | ≤ 62 dB | Must be ≤62 dB — the lower the value, the better the impact sound insulation |

*There is NO Part E impact sound requirement for separating walls* — impact sound in walls (e.g., banging on a party wall) is covered by the airborne DnTw standard.

**Two compliance routes under Part E**:

There are two routes to demonstrating compliance with Part E for new-build dwellings and new-build portions of conversions:

*Route 1 — Pre-completion acoustic testing (PCT)*: The separating walls and floors are acoustically tested (measured) after construction but before the dwelling is occupied, to demonstrate that the required DnTw and L'nTw values are achieved.

*Route 2 — Robust Details (RD)*: The design and construction matches a pre-approved Robust Details specification, which has been shown by testing to achieve compliance without site testing on every individual dwelling. The Robust Details scheme allows builders to register dwellings on the Robust Details website (rbdetails.co.uk) and avoid site acoustic testing, providing the construction matches the registered detail exactly.

• *Key difference*:
• •PCT: requires measurement after construction; any failure requires remediation and re-testing (costly and time-consuming)
• •Robust Details: no site testing (provided construction matches the detail exactly); registration fee per dwelling (approximately £19.20 + VAT per plot in 2025); significantly reduces risk of failure

For London flat conversions and new-build residential above garages or commercial units, Robust Details are the standard industry approach. Pre-completion testing is used when a non-standard specification is chosen or where Robust Details are not available for the specific construction type.

**The Robust Details scheme — how it works for London conversions**:

• The Robust Details scheme (operated by Robust Details Ltd, a subsidiary of the House Builders Federation) publishes a library of pre-tested separating wall and floor construction specifications — the 'Robust Details' — each identified by a code:
• •**E-type details (E-WM-x.x; E-FC-x.x; E-WT-x.x)**: for new-build houses — separating walls (masonry; timber frame) and separating floors
• •**H-type details (H-WM-x.x; H-FC-x.x; H-WT-x.x)**: for conversions — house-to-flat conversions; loft conversions creating new flats above existing flats

The most relevant Robust Details for London house-to-flat conversions:

*Separating walls — conversion (H-type):*

| Robust Detail code | Description | Typical application in London | |---|---|---| | H-WM-1 | Masonry cavity wall with independent masonry leaf on one side | New masonry separating wall in a conversion where sufficient space allows a 100mm cavity or independent leaf | | H-WT-1 | Timber frame with resilient bar treatment on both faces | New timber stud separating wall in a conversion where space is very limited — typical for London Victorian flat conversions where room sizes are tight | | H-WT-2 | Timber frame with double leaf independent frame construction | Higher performance timber stud wall — used where the standard WT-1 may be marginal |

*Separating floors — conversion (H-type):*

Separating floor Robust Details for conversions are the most important and technically complex. A typical Victorian terrace first floor (which becomes the separating floor between a lower flat and an upper flat in a house-to-flat conversion) needs to be upgraded to meet the Part E standard:

| Robust Detail code | Description | Typical application | |---|---|---| | H-FC-1 (Platform floor — resilient layer) | Existing timber floor structure; new floating platform floor above (mineral wool between joists; resilient layer on top of structural boarding; screed or floating floor finish) | Most common specification for Victorian terrace flat conversion — the most accessible approach using the existing joists | | H-FC-2 (Platform floor — isolated ceiling below) | Existing timber floor structure; new floating ceiling below (resilient bars; additional plasterboard layers) | Where floor height allows raising the floor; or where ceiling height below permits dropping the ceiling | | H-FC-3 (Independent ceiling below) | Existing timber floor structure; new independent ceiling below (on a separate structural frame below the existing joists; mineral wool fill) | Highest performance; used where H-FC-1/2 insufficient; loses significant ceiling height in lower flat |

• *Critical: flanking transmission*:
• Flanking transmission is sound that bypasses the separating element (wall or floor) by travelling through connected structural elements — the party wall into the floor joists; the floor joists into the wall; the wall into the ceiling. In Victorian terraces, flanking transmission via the party wall is a major acoustic challenge:
• •The solid brick party wall (225mm) is structurally continuous from foundation to roof — it flanks both the separating floor and separating wall
• •Flanking transmission is the reason why simply installing acoustic matting on a Victorian terrace floor is not sufficient to pass the Part E test — the acoustic path through the party wall and flanking structure must also be addressed
• •Robust Details H-FC-1 and H-FC-2 include specific flanking measures: the floating floor must be isolated from all walls (including the party wall) by a resilient perimeter strip; the ceiling must not be fixed through to the party wall

**Acoustic construction details for London house-to-flat conversions**:

*Separating floor upgrade (Victorian terrace — H-FC-1 Platform Floor approach)*:

The standard specification for converting the first floor of a London Victorian terrace into a Part E-compliant separating floor:

1. **Existing joist void**: fill the spaces between existing timber floor joists with unfaced mineral wool quilt (minimum 100mm at 60 kg/m³ density — higher density than standard thermal mineral wool; acoustic mineral wool is specifically designed for sound attenuation; Rockwool RW3 or Rockwool RWA45; or Knauf Rocksilk Acoustic Slab). This fills the air void through which airborne sound transmits as a direct bypass path

2. **Existing floorboards**: these can typically remain in place (fixed to the joists) — they form the substrate for the floating floor platform above

3. **Resilient layer**: a high-density rubber or cork or composite resilient layer (typically 5–25mm thick; specific product from the Robust Details approval — e.g., Regupol 4515 Impact; Regupol BSW; or Farrat Acoustics Taper-Loc) is laid over the existing floorboards. This layer provides impact sound reduction by decoupling the floating floor from the structural floor

4. **Structural platform (floating floor)**: 18–22mm T&G chipboard or plywood (two layers at 90° to each other for H-FC-1 specifications) is laid over the resilient layer. This creates the structural floating platform

5. **Perimeter isolation**: the floating floor platform must be isolated from ALL walls (including the party wall) by a strip of the resilient material around the perimeter. This prevents flanking: if the floating floor touches the party wall, impact sound travels from the floor through the wall regardless of the floating floor specification

6. **Ceiling treatment below**: the existing ceiling in the lower flat is typically supplemented by one or more layers of sound-reducing plasterboard (typically 15mm Type F or 12.5mm + 15mm double-layer with resilient bar) to increase the airborne sound DnTw. For H-FC-1, the ceiling may be left in its existing state if the combined airborne performance is predicted to be adequate — but adding resilient bar ceiling treatment significantly improves performance and is almost always recommended for London Victorian conversions where the airborne Part E target (DnTw ≥45dB) may be marginal

*Separating wall specification (where a new separating wall is created in a conversion)*:

For a London house-to-flat conversion where a new separating wall is created (dividing a hallway; creating a private entrance lobby): H-WT-1 Timber frame with resilient bars:

1. 89mm or 100mm stud frame (C16/C24 timber) 2. 75–100mm acoustic mineral wool quilt (60 kg/m³; Rockwool RWA45 or equivalent) filling the stud void completely 3. Resilient bars (e.g., Gypframe RB1 resilient bars at 600mm centres, horizontal) fixed to the studs on BOTH faces of the wall 4. Two layers of 12.5mm standard plasterboard or one layer of 15mm high-density plasterboard on each face, fixed to the resilient bars (never screwed direct to studs — the resilient bars are the decoupling element) 5. Joints staggered between layers; joints taped and filled 6. Perimeter sealed with acoustic sealant (Geocel Trade Mate Acoustic Sealant; or PU acoustic sealant) at all junctions with floor, ceiling, and walls — air leakage paths reduce acoustic performance significantly

**Acoustic testing — when pre-completion testing is required and what it involves**:

Where Robust Details are not used (or where a non-RD specification is chosen), pre-completion acoustic testing (PCT) is required by Building Control. Testing must be carried out by a UKAS-accredited acoustic testing laboratory using calibrated equipment.

*PCT process*: 1. Testing is carried out after all construction is complete (including floating floors; ceiling finishes; walls completed) but before carpets, furniture, or the dwelling is occupied 2. A standardised noise source (a calibrated dodecahedral loudspeaker — a 12-sided speaker) is placed in the source room; a sound level meter is placed in the receiving room; multiple measurements are taken with the speaker in multiple positions; results are averaged 3. Impact sound is measured by a standardised tapping machine (a calibrated mechanical device with 5 hammers that tap the floor at 10 impacts per second) 4. Results are analysed by the acoustic laboratory and reported as DnTw (airborne) and L'nTw (impact) values 5. If results meet Part E: Building Control issues the completion certificate 6. If results fail Part E: remediation works are required (typically adding resilient ceiling treatment below; additional floating floor layers; sealing flanking paths) followed by re-testing

*Pre-completion testing costs in London (2025)*: approximately £400–£800 for a single flat-to-flat test (airborne wall + floor; impact floor). Robust Details registration (alternative): approximately £19.20 + VAT per plot (significantly cheaper than testing; eliminates risk of test failure).

**Sound insulation in HMO conversions — where Part E applies and what to do**:

For HMO conversions in London (typically a 4–6-bedroom house being converted for multi-occupancy rental), Part E applies where the conversion creates new separating elements between dwellings. However, the interpretation of this in an HMO is nuanced:

*HMO where the building is still classified as a single dwelling (HMO — single dwelling)*: If the HMO is licensed as a house (not as separate self-contained flats), it is technically a single dwelling for Building Regulations purposes. In this case, Part E does NOT apply to the internal walls between bedrooms — they are internal walls within a single dwelling (not separating walls between separate dwellings). However, many London HMO landlords install improved acoustic insulation between bedrooms as a quality measure — it reduces complaints between tenants and can improve tenant retention.

*HMO where individual rooms are self-contained units with locked doors (quasi-flat)*: If individual rooms are clearly marketed and let as self-contained units (with their own bathroom; kitchenette; locked entrance), Building Control may classify the rooms as separate dwellings — in which case Part E applies to the walls and floors between them. The legal distinction between a single HMO and separate self-contained units is important — seek specialist advice if in doubt.

**Good-practice acoustic specification for HMO bedrooms (recommended even where Part E is not mandatory)**:

• *Between HMO bedrooms (party walls)*:
• •Double-stud timber frame (two rows of 50mm studs with a 50mm gap between them; no rigid connection between the two frames) — higher performance than a single-stud resilient-bar wall
• •75–100mm Rockwool RWA45 in each stud cavity
• •Two layers of 12.5mm or 15mm plasterboard on each face
• •Perimeter sealed with acoustic sealant

• *Between HMO floors*:
• •Acoustic mineral wool between joists (Rockwool RWA45; 60 kg/m³)
• •Resilient layer floating floor (Regupol or similar)
• •Resilient bar ceiling treatment below (Gypframe RB1 at 600mm centres)
• •Double plasterboard ceiling (15mm + 12.5mm)

**Sound insulation in London loft conversions — the party wall challenge**:

A loft conversion above a London Victorian terrace creates a new habitable room immediately above the existing first-floor ceiling. The existing first-floor ceiling becomes the floor of the loft room. Part E does NOT apply to the loft floor in a single-occupancy house (it is an internal floor within a single dwelling). However, the acoustic performance of the loft floor is important from a quality-of-life perspective — footfall and impact sound from the loft room can be clearly audible in the first-floor bedrooms below.

• *Recommended loft floor acoustic specification for London Victorian terrace loft conversions*:
• •New loft floor joists: use acoustic resilient floor clips or acoustic joist hangers to decouple the loft floor from the supporting walls (reducing flanking via the walls)
• •Acoustic mineral wool between the new loft floor joists (75mm; Rockwool RWA45)
• •Acoustic floating floor system (two layers of 18mm T&G chipboard with a resilient interlayer; or a proprietary acoustic floating floor such as Regupol Sound, IsoFloor, or Soundtect)
• •Existing ceiling below: do not penetrate or alter the existing first-floor ceiling where possible — the existing lath and plaster ceiling (common in Victorian terraces) has reasonable acoustic mass and should be retained where structurally sound

**Party wall sound insulation — the most common acoustic complaint in London terraces**:

• The 225mm solid brick party wall in a Victorian London terrace provides approximately 45–52 dB DnTw airborne sound insulation in its existing untreated condition — broadly meeting the Part E standard of DnTw ≥45 dB for the wall element alone. However, this figure applies only to airborne sound passing directly through the wall — it does not account for:
• •Flanking transmission via connected floor, ceiling, and wall structures
• •Gaps or cracks in the party wall (particularly where old chimney breasts have been removed and the wall has been poorly made good)
• •Noise paths via windows facing the shared courtyard or rear garden

• *Improving acoustic performance of an existing Victorian party wall (during a refurbishment)*:
• •Independent internal wall lining (an independent stud frame built 25–50mm away from the face of the party wall; with acoustic mineral wool in the cavity; and double plasterboard finish). This adds 10–15 dB to the existing wall DnTw and addresses flanking transmission
• •Cost in London (2025): approximately £60–£120/m² (supply and install; acoustic mineral wool; resilient bars or independent frame; double plasterboard; skim)
• •This is NOT a Building Regulations Part E obligation for a single-dwelling refurbishment — but is a voluntary upgrade that significantly improves acoustic quality, particularly in London terraces where neighbour noise is a common complaint

**Extensions adjacent to a neighbour's property — when Part E applies to the extension**:

A new London house extension that is entirely within the building owner's property (no new separating wall or floor between two dwellings) does NOT require Part E compliance — it is a new space within a single dwelling. However, where an extension creates a new floor or wall that separates two dwellings (for example, extending above a neighbour's flat; or building a two-storey extension that creates a new floor above a tenanted basement flat), Part E applies to that new separating element.