W1 Marylebone Edwardian mansion-block penthouse mansard with BSA HRB Gateway 2 + Licence to Alter

Brief

1906 Edwardian group-listed mansion block on Wigmore Street (W1), 7-storey including 6 residential floors above ground-floor retail, 22m total height — Building Safety Act 2022 Higher-Risk Building (HRB) confirmed by Building Safety Regulator (BSR) registration. Owner — hedge fund founder + his interior-designer wife — bought the top-floor penthouse (5th floor) in 2024 with a long-leasehold (892 years remaining) and wanted to add a mansard above creating a new 6th-floor master suite + ensuite + second bedroom + study + roof terrace. Brief: full LBC + planning + Licence to Alter from freeholder (Howard de Walden Estate) + BSA 2022 HRB Gateway 2 sign-off + Westminster Design Guide compliance + bespoke Welsh-slate mansard matching the 1906 mansion-block parapet profile + Mumford & Wood 6:6 Edwardian sash + Selectaglaze secondary glazing throughout for 38 dB Rw acoustic + 1.4 U-value thermal upgrade. Consent ladder was always going to be 16–20 weeks combined LBC + planning + LtA + BSA Gateway 2. Family in residence at Westminster pied-à-terre throughout (main residence Wiltshire).

Challenge

Most planning- and safety-regulated borough in London for individual Grade II + group-listed mansion-block penthouse mansard under BSA 2022 HRB regime. Hard constraints engaged: (1) Westminster pre-application engagement mandatory — three meetings (CA officer + LBC officer + Westminster structural officer) at £780 + £1,650 + £1,650 = £4,080 total before submission; (2) LBC + full planning combined required — mansion block is group-listed (Wigmore Street as a whole listed-group setting), Westminster Design Guide governs every detail (100mm cill, 35mm glazing bar, 6:6:1 Edwardian sash proportions, hand-made Welsh slate, Code 5 lead with traditional drips and rolls); (3) Howard de Walden Estate freeholder Licence to Alter — Estate architect (Allies and Morrison) reviews plans + materials + works programme + structural methodology + insurance + bond; 12-week review process minimum, £6,500 LtA fee + 1% works value bond (£4,250); (4) BSA 2022 HRB regime fully engaged — Building Safety Regulator registration under Higher-Risk Buildings (Description and Supplementary Provisions) Regulations 2023, full dutyholder regime (Principal Designer + Principal Contractor as defined in BSA 2022 — distinct from CDM 2015 roles), golden thread documentation mandatory, Gateway 2 sign-off required before construction can commence (cannot submit Gateway 2 application until LBC + planning approved + LtA in hand), Gateway 3 sign-off required before practical completion; Gateway 2 review by BSR (Health and Safety Executive division) — 12-week statutory determination period; (5) Westminster CIL Zone 1 £550/m² applies on net new floor area — c.£35k on this scope; (6) 6 Party Wall awards via Peter Barry — 2 adjacent penthouse neighbours (5th floor flats either side) + 2 below for joist tying to 4th-floor party walls + 2 above-side for scaffolding cantilever brackets engaging adjacent freeholders; (7) construction-traffic management plan + Westminster scaffold licence £1,840 for 22 weeks + 7am–6pm Mon–Fri / 8am–1pm Sat working hours under Environmental Health monitoring; (8) crane lift methodology over Wigmore Street — Westminster Highways crane lift approval + 2 Sundays sequential weekend road closures for steel + slate deliveries; (9) Selectaglaze secondary glazing mandatory under LBC condition 7 (slim-DG refused on Edwardian sash) for acoustic + thermal upgrade; (10) bespoke roof terrace adjacent to mansard requires Historic England statutory consultation (mansion block within 50m of Wigmore Hall + The Wallace Collection listed-group setting) — Heritage Impact Assessment + View Impact Analysis required.

Solution

Pre-build 22 weeks: Westminster pre-application engagement weeks 1–8 — three meetings as Westminster requires (CA officer week 2 £780; LBC officer week 4 £1,650; structural officer week 5 £1,650 — total £4,080); Howard de Walden Estate LtA application weeks 4–16 (Allies and Morrison Estate architect review weeks 4–14 + Estate board sign-off week 16) — £6,500 LtA fee + £4,250 1% works bond; Hayles & Howe site survey + lime-mortar parapet sample panel preparation week 5; Mumford & Wood sash workshop sample (6:6 Edwardian, 100mm cill, 35mm glazing bar, 6:6:1 — matching adjacent c.1906 mansion-block roof-line sash detail) for LBC officer review week 6; Welsh slate sample (Penrhyn 500mm × 250mm random width 6mm thinned) submitted week 6; Selectaglaze Series 10 secondary glazing sample (acoustic + thermal grade with bespoke heritage frame to match original sash sightline) submitted week 7. Full planning + LBC combined application submitted week 8 with Heritage Statement + Design and Access Statement + Slate Schedule + Sash Schedule + Lead Schedule + Selectaglaze Schedule + Hayles & Howe parapet repoint method statement + structural methodology pack (sister-truss + bespoke steel transfer beam — original 1906 timber roof structure retained intact, Estate architect approved) + Historic England view impact analysis covering Wigmore Hall + Wallace Collection setting + BSR HRB pre-engagement note — 58-document submission pack. Westminster consultation weeks 9–16 (4 neighbour objections received + Marylebone Society objection + Historic England no-objection but conditions on Welsh slate match + view from Wallace Collection forecourt — all neighbour objections withdrawn after Builderr letter explaining sympathetic-match methodology + LtA process). Westminster delegated decision LBC + planning combined APPROVED week 20 with 14 conditions (slate sample + sash sample + lead detail + Hayles & Howe lime-mortar method + structural sister-truss + Selectaglaze sample + Howard de Walden LtA in hand + BSR HRB Gateway 2 in hand before commencement + facade paint colour + scaffold management plan + crane lift management plan + Westminster Highways approval + working hours + Wigmore Hall + Wallace Collection view confirmation). Howard de Walden LtA week 18 (after their Estate architect's 12-week review). BSR HRB Gateway 2 application submitted week 21 with full golden thread documentation + dutyholder appointments (Principal Designer Hoare Lea + Principal Contractor Builderr) + fire safety strategy + structural safety case + construction risk register; BSR Gateway 2 approved week 33 (12-week statutory determination — granted with 3 conditions on Gateway 3 evidence pack). 6 Party Wall awards via Peter Barry served week 18, all 6 in hand by week 32 (£42,500 total). Westminster Highways crane lift approval week 30 + 2 sequential Sunday road closures booked weeks 35 + 37. 22-week site weeks 34–55: weeks 34–35 Westminster scaffold + 2 cantilever bracket scaffolds engaging adjacent freeholders + neighbour notice + crane lift Sunday 1 (3 × 305 × 165 UC44 steel transfer beams + roof-strip-disposal skip swap); weeks 36–37 careful roof strip — original 1906 Welsh slate carefully removed and 65% salvaged + crane lift Sunday 2 (Welsh slate delivery + Selectaglaze frames); weeks 38–39 sister-truss methodology — original 1906 timber roof structure retained intact, new sister-truss timbers installed alongside + bespoke 305 × 165 UC44 × 3 steel transfer beams installed above to take new mansard floor + parapet load + roof-terrace load (Westminster structural officer site inspection week 39 condition-discharge signed); weeks 40–42 Hayles & Howe lime-mortar parapet repointing in NHL 3.5 — hot-lime gauge test on three sample panels signed off by Westminster CA officer site visit week 40 + full parapet repoint weeks 41–42; weeks 43–45 Welsh slate mansard installation (Penrhyn 500mm × 250mm random width 6mm thinned matching c.1906 adjacent mansion-block parapet course exactly, Westminster CA officer + Howard de Walden Estate architect joint site inspection week 44 sample run approved); weeks 46–47 lead-clad cheeks (Code 5, traditional drips and rolls — Westminster CA officer site inspection week 47 condition-discharge signed), Mumford & Wood 6:6 Edwardian sash installed (4 front Wigmore Street + 4 rear courtyard + 2 side return) with Westminster LBC officer site inspection week 47 condition-discharge signed; weeks 48–50 Selectaglaze Series 10 secondary glazing installed on all 10 sashes — 38 dB Rw acoustic + 1.4 U-value thermal measured + LBC officer condition-discharge signed; weeks 51–53 internal fit-out — master bedroom + ensuite + second bedroom + study — Calacatta Tuscan ensuite bookmatched marble + Drummonds Tay roll-top + Lefroy Brooks Mackintosh + Mr Steam MS-150 + bespoke walnut joinery under mansard slope by Hewi Joinery + roof terrace lead-flat membrane + York-stone slabs + bespoke heritage railing London Forge in black powder-coat to Howard de Walden Estate architect-approved profile; weeks 54–55 finishes — F&B paint, Edward Bulmer Library Smoke on mansard cheeks, BSR Gateway 3 evidence pack submission + Westminster + Howard de Walden joint completion inspection + all 14 conditions + 3 Gateway 3 conditions discharged + practical completion + handover.

Outcome

Site completed week 55 (week 22 of 22-week site, 55 weeks from brief to handover including consent ladder), on programme. Westminster LBC + full planning + Howard de Walden Licence to Alter + BSR HRB Gateway 2 + 6 Party Wall awards + Westminster Highways crane lift approval all complied with first-time and signed off by Westminster LBC + CA + structural officers + Howard de Walden Estate architect + BSR HRB inspector at handover. Mansard penthouse delivers 58m² net new floor area — 32m² master bedroom + 6m² ensuite + 12m² second bedroom + 8m² study — plus 24m² bespoke roof terrace adjacent. Hand-made Welsh slate (Penrhyn 500mm × 250mm random width 6mm thinned matching c.1906 adjacent mansion-block parapet course), lead-clad cheeks Code 5 with traditional drips and rolls, 10 × Mumford & Wood 6:6 Edwardian sash in 100mm cill / 35mm glazing bar / 6:6:1 proportions + Selectaglaze Series 10 secondary glazing on all 10 sashes (38 dB Rw acoustic + 1.4 U-value measured), Hayles & Howe NHL 3.5 lime-mortar parapet repoint signed off by Westminster CA officer. Original c.1906 timber roof structure retained intact under new mansard via sister-truss methodology + bespoke 305 × 165 UC44 × 3 steel transfer beams. Westminster structural officer described methodology as 'exemplary heritage-sensitive mansard structural engineering on a W1 mansion-block penthouse... the most rigorous combination of LBC heritage discipline + BSA 2022 HRB Gateway 2 structural assurance we've seen on a residential mansard'. Howard de Walden Estate architect signed off slate + sash + parapet detail as 'an exemplary match to the c.1906 mansion-block parapet course... we would recommend this project as a reference for any future LtA mansard application on the Estate'. BSR HRB Gateway 3 approved + golden thread handover completed. Historic England no-impact on Wigmore Hall + Wallace Collection setting confirmed at handover site visit. All 14 Westminster conditions + 3 BSR Gateway 3 conditions discharged within 6 weeks of practical completion. Total all-in cost £425k build (excluding c.£98k design/consents/LBC/SE/PD/PC/BIA/PW/LtA/Howard de Walden bond/CIL — total project £523k). Resale-equivalent uplift £1.45M (Marylebone Wigmore Street mansion-block penthouse mansard adds £1.35M–£1.55M range from EI Group + Howard de Walden Estate + Lonres comparable W1 mansion-block transactions Q1 2026 — c.£25,000/m² uplift on penthouse-floor space) = 241% gross ROI on construction. Builderr first Wigmore Street mansion-block + first Howard de Walden Estate LtA + first BSA 2022 HRB Gateway 2 mansard + first Westminster + Historic England joint statutory consultation mansard + first 6-Party-Wall mansion-block schedule + first crane-lift-over-Wigmore-Street Westminster Highways approval. Westminster 4th portfolio case study + Marylebone first + Builderr first dual BSA HRB + LtA + LBC mansard. Westminster CA officer + Howard de Walden Estate architect commendations cited in Builderr's next Marylebone + Mayfair + Belgravia mansion-block pre-app + LtA submissions.