Indicative planning allowance for a straightforward 20–40m² single-storey rear London extension. Not a guaranteed BCS quote.
This opens the contact page on the BCS Builders London website. Your figures are not sent with it.
This is a planning allowance for the common core build only, not a BCS guaranteed quote or a full project budget. It excludes kitchen and other fit-out, decoration, floor finishes, works to the existing house, architect, engineer, planning, building control and party wall costs, and abnormal ground, structural or access conditions. Add those separately. The low-to-high spread does not show how likely any figure is, and the real cost may fall outside it.
The rates exclude VAT. The VAT figure is an illustration at the standard 20% rate on the total allowance (about £3,360–£3,720/m² before contingency). It is not an invoice or a statement of tax owed. Whether VAT applies depends on the supplier and the work. The reserve is a budget amount, so VAT shown on it is illustrative only.
A straightforward, non-luxury single-storey rear extension of 20–40 m² on an ordinary, accessible site, with standard construction: foundations, walls and structure, roof, standard external glazing, first-fix services and plastering.
Kitchen, bathroom and other fit-out; decoration and floor finishes; works to the existing house; architect, engineer, planning, building control and party wall costs; abnormal ground, structural or access conditions. Some published sources include more than this, but the benchmark does not.
The rates exclude VAT. The VAT figure is an illustration at the standard 20% rate on the total allowance (about £3,360–£3,720/m² before contingency). It is not an invoice or a statement of tax owed. Whether VAT applies depends on the supplier and the work. The reserve is a budget amount, so VAT shown on it is illustrative only.
Sources checkedBase allowance = area × rate, worked out separately for the low and high rates and rounded to the nearest penny, with halves rounded up. Contingency reserve = base allowance × contingency ÷ 100, rounded the same way. Total allowance = base allowance + contingency reserve, all excluding VAT. The VAT illustration is 20% of the total allowance, rounded the same way, and is added to give the total including VAT. All calculations use whole-penny arithmetic. Method dated 15 October 2026. It does not update automatically.
It covers only straightforward single-storey rear extensions of 20–40 m² on an ordinary, accessible site, and only the common core build. It does not check whether rates you enter are current or cover the same scope. It does not assess risk, so the contingency is a starting figure you can change. The VAT figure is an illustration at 20%, not an invoice or a statement of what VAT you would pay. For larger, smaller, multi-storey or unusual projects, get a project-specific quote.
The starting rates are a benchmark chosen by BCS Builders London from published London guides, not BCS's own price list. If you replace them with rates you were given, take both from the same source, for the same scope and VAT basis, and enter their date. The scope and exclusions shown here stay the same whichever rates you use.
Talk to BCS Builders London about your extensionSources checked 15 October 2026. Prices change, so check the date.
Four published London guides give explicit, non-luxury construction bands excluding VAT: £2,800–£3,200, £2,500–£3,100, £2,550–£3,450 and £2,500–£3,200/m². £2,800–£3,100/m² is the range common to all four. Their labels (basic, standard, mid-range) and finish details differ, so we limited the scope to the core they share. This is an editorial choice, not an average, a survey or a like-for-like tender comparison. Three other sources were checked but left out because their scope is not comparable.
Used. Basic build-only London rate of £2,800–£3,200/m² excluding VAT and professional fees: shell, standard openings, first-fix services and plastering. Fit-out, kitchen, flooring and decoration excluded. 2026 guide; wording on finishes and glazing is not fully consistent.
Used. Standard construction London rate of £2,500–£3,100/m² excluding VAT and fees, with richer finishes than Build Team, including heating and board flooring. Kitchen excluded. 2026 guide.
Used. Standard single-storey rear London rate of £2,550–£3,450/m² excluding VAT and professional fees, from the page's rear-extension FAQ. Includes groundworks, shell, roof, windows and doors, plastering, first-fix services, floor finishes and preliminaries. Updated May 2026.
Used. Mid-range rear extension London rate of £2,500–£3,200/m² excluding VAT, with bi-fold or sliding doors, a roof lantern and good-quality finishes. Kitchen, fees and landscaping excluded. Its lower standard tier of £2,000–£2,500/m² is noted as counter-evidence. 12 April 2026.
Not used in the range. Its structural envelope rate from £2,125/m² excluding VAT leaves out supplied glazing and internal trades, so it is not comparable with its finished trade tier from £3,325/m² or with the benchmark scope. Reviewed September 2026.
Not used in the range. Its 18 m² example gives £52,000 including VAT (£2,890/m²) in one place and a £55,000 table total in another, and it excludes supply and fitting of doors and windows. August 2026.
Not used in the range. Its single-storey rate of £2,300–£3,000/m² plus VAT is for design and build, including design, planning and regulations, so it is not a build-only rate. 2026 guide.
How to work out the new internal floor area in m² for the calculator, including L-shapes.
What the £2,800–£3,100/m² benchmark covers, and how to compare it fairly with a rate or quote you've been given.
Why the calculator starts at 10%, and how to decide whether to change it.