When security can be required
s12(1) Party Wall Act 1996 grants the adjoining owner the right to require security for expenses before the building owner commences any works under the Act. The amount + form are determined by the appointed surveyors (or third surveyor) if not agreed. Typical trigger threshold = where works pose material risk of damage + cost of remediation could exceed contractor's contract insurance + building owner's personal worth. Most commonly required in: (1) basement construction (high-risk lateral movement, complex underpinning, settlement risk on adjoining buildings) — security £50,000–£500,000+ typical for prime central London; (2) significant underpinning of shared party walls; (3) where building owner is an SPV/shell company with limited assets — adjoining owner concerned recovery against limited-liability entity inadequate; (4) where building owner is overseas-resident or judgment-proof; (5) where contractor's PL insurance is below £5M (atypical); (6) where adjoining property is high-value heritage (Grade II listed, Kensington terrace, etc.) + repair would exceed insurance recovery. Standard rear extension on private-individual building owner with creditworthy contractor: security rarely required.
Amount + form
Surveyors assess security amount based on (a) reasonable foreseeable cost of remediation if worst-case damage occurs — typically structural movement + cosmetic repair + decorations + temporary accommodation if uninhabitable; (b) adjoining property value (higher-value buildings = higher repair costs); (c) works risk profile (basement > deep extension > standard extension); (d) contractor insurance + building owner net worth backstop. Typical figures for central London: standard extension security £15,000–£45,000 (rare); side-return + deep foundation security £25,000–£75,000; basement security £75,000–£250,000+; prime PCL basement security £250,000–£1,000,000+. Form (s12(2) Act options): (1) cash deposit held in joint escrow account at solicitor or surveyor firm; (2) bank guarantee from building owner's bank (cleaner but bank fee 0.5–1.5% pa); (3) insurance bond from specialist surety bond provider (e.g. CIS Security, Liberty Bond — fee 1.5–3% of bond value pa); (4) solicitor's undertaking (rare, only where solicitor holds equivalent client funds). Cash deposit most common for amounts under £100k; bond more practical above. Bond fees become significant: £200k security at 2% pa for 18-month build = £6,000 cost to building owner.
Process + release
Process: (1) adjoining owner serves request for security via their appointed surveyor; (2) surveyors negotiate amount + form (jointly or refer to third surveyor if disagreement); (3) Award includes security clause specifying amount, form, holder, release conditions; (4) security deposited before works can lawfully commence — building owner cannot start until security in place; (5) during works, security cannot be drawn against without joint surveyor sign-off (typically requires emergency damage repair); (6) on completion, post-works re-inspection of adjoining property compared to Schedule of Condition; (7) where no damage or damage repaired + paid, security released to building owner with interest accrued. Timeline: security setup adds 4–8 weeks to pre-start programme — solicitors document escrow, bank issues guarantee, bond surveys building owner financials. Build into RIBA Stage 4 programme. Cost: solicitor fees £950–£3,500 for escrow setup; bank guarantee fee 0.5–1.5% pa; bond fee 1.5–3% pa. Cashflow impact: £100k+ cash tied up for 12–24 months is real opportunity cost — finance via bond preferable on large schemes to preserve build cashflow. Builderr practice: anticipate security at design stage for basement + deep foundation schemes; factor into client financial planning; engage bond broker early to optimise cost.
