Honest, plain-English answers to the questions homeowners ask us every week — costs, planning, party walls, building control, insurance and timelines. Written by the team at RCB Design & Build.
In 2026 a standard dormer loft conversion in London typically costs £55,000–£90,000. A mansard (full-rear, vertical wall) sits at £80,000–£110,000, and an L-shaped dormer on a Victorian terrace is usually £75,000–£105,000. Prices vary with finish, en-suite, structural steels and party-wall complexity. A free site visit and full drawings are required for a binding quotation.
In most cases a single-storey rear extension up to 6 metres (terraced/semi) or 8 metres (detached) is covered by Permitted Development under the Neighbour Consultation Scheme — no full planning is needed but you must apply for Prior Approval. Two-storey rear extensions, conservation areas, listed buildings and many flats always need full planning permission. A Lawful Development Certificate is strongly recommended even for PD-covered work.
A full strip-out refurbishment of a 3-bed terrace typically takes 12–16 weeks on site, plus 2–4 weeks of design and procurement upfront. A flat refurbishment is usually 8–12 weeks. Add 4–8 weeks if the works include structural alterations (steel beams, opening up) or a kitchen extension running in parallel.
A reputable UK contractor carries (a) Public Liability of at least £5 million, (b) Employer’s Liability of £10 million (legally required if they employ anyone), (c) Contract Works insurance covering the project itself, and (d) Professional Indemnity if they provide design. Ask for copies of certificates before signing. RCB Design & Build is FMB, TrustMark, Checkatrade and TrustATrader accredited with full cover.
A scope of works (SoW) is the line-by-line document that defines exactly what the contractor will and will not do — every trade, every material, every inclusion and every exclusion. A good SoW prevents 95% of the disputes that happen on residential projects. RCB issues a full SoW with every estimate over £15,000 and shares it with the client to review before pricing is added.
For a typical 3-bed semi a rear scaffold (loft conversion or rear extension) costs £1,800–£3,500 for the first 4 weeks plus £150–£300 per week extension. A full-wrap (whole-house) scaffold for re-roofing or render is usually £3,500–£7,000. Pavement licences (if scaffolding sits over the public highway) add £200–£600 depending on borough.
You need a Party Wall Notice (and almost always an Award) if you are cutting into, building on or excavating within 3 metres of a shared wall, or excavating within 6 metres if foundations go deep. Most loft conversions, side extensions and structural opening-up trigger it. Allow 2 months for the process and £900–£2,000 per neighbour in surveyor fees.
Building Control inspects structural, fire, energy and safety compliance under the Building Regulations. You need a Building Control application for almost any structural work — extensions, loft conversions, structural openings, new bathrooms, electrical alterations (Part P), drainage changes and most insulation upgrades. RCB lodges the application on your behalf at the start of every project; fees are typically £600–£1,800 depending on the council and the project value.
Get a free, no-obligation site visit and a fixed written quotation within 5 working days.