Renovation Contingency Budget: How Much to Hold and Why

• **The 10% rule — where it comes from and when it is appropriate**:
• The most commonly quoted contingency figure in residential construction is 10% of the total project cost. This figure comes from commercial construction practice, where:
• •Projects typically have full design documentation before tender
• •Scope is well-defined before work starts
• •Existing site conditions are known (ground investigations, structural surveys, MEP surveys)
• •The contractor's tender is based on complete information

In well-defined, well-documented commercial projects, 10% is a reasonable contingency. But residential renovation and extension work in London — particularly in Victorian and Edwardian properties — is rarely well-documented before work starts, and the existing building almost always contains surprises.

**Why 10% is insufficient for older London properties**:

*Unknown existing conditions*: A pre-1919 London property was built without engineering drawings, without a ground investigation, and without a materials schedule. Until walls are opened, ceilings are dropped, and floors are lifted, the condition of the structure, the services, and the substructure is unknown. The Building Regulations at the time did not require insulation, damp courses, ventilation, or structural engineer oversight — so the baseline condition of the building can be far below modern standards.

• *Specific risks in Victorian and Edwardian properties*:
• •Lead water supply pipes (require replacement — cost: £2,000–£8,000)
• •Knob-and-tube wiring or rubber-insulated wiring (full rewire required — cost: £4,000–£12,000)
• •No DPC or failed DPC (treatment required — cost: £1,500–£6,000)
• •Lathe-and-plaster in poor condition requiring full replacement rather than skim (cost: £25–£45/m² for hack-off and replaster)
• •Chimney breasts removed without proper support of the breast above (hidden structural defect — cost to rectify: £3,000–£10,000)
• •Asbestos in artex, floor tiles, pipe lagging (licensed removal — cost: £1,000–£8,000)
• •Undersized floor joists (engineered joist sistering required — cost: £3,000–£9,000)
• •Failed or absent rainwater drainage (blocked, broken, or unvented underground drainage — cost to rectify: £2,000–£12,000)
• •Ground conditions worse than anticipated (old well, buried rubble, soft spots in clay — cost to rectify: £5,000–£30,000+)

**Revised contingency rules by project type**:

*New extension on a modern property in known condition*: 10% contingency is appropriate — the scope is defined, the existing building is known, and the main risk is minor price changes and material specifications.

*Extension on a Victorian or Edwardian terrace without prior structural survey*: 15–20% contingency is appropriate. The risk of discovering structural defects, failed services, and unknown substructure conditions is high.

*Full house refurbishment of an unlived-in or long-unoccupied Victorian property*: 20–25% contingency is the minimum appropriate figure. Properties that have not been maintained for years, that have been stripped of fittings, or that have been left unheated are the highest-risk category — every system may be compromised, and the building fabric condition is unknown until walls are opened.

*HMO conversion or flat conversion in a Victorian terrace*: 15–20% contingency — the combination of unknown building condition and the compliance requirements (fire safety, acoustic performance, gas/electrical certification) creates multiple parallel risk items that can compound.

*New loft conversion (Velux, dormer) on a well-maintained property*: 10–15% contingency — the roof structure may be in better or worse condition than expected; the staircase location may require unforeseen structural work; the floor structure may require upgrading beyond what was anticipated at planning stage.

Contingency is not a tolerance for poor estimating or a discount fund — it is a defined reserve for specific types of unforeseen cost. Managing it correctly requires being specific about what it covers.

**What contingency should cover**:

• *Category 1 — Unforeseen existing conditions (the primary contingency use)*:
• •Discovery of failed, absent, or inadequate existing structure requiring remediation before the new work can proceed
• •Discovery of services in a condition worse than assumed at estimate stage (lead pipes, rubber-insulated wiring, inadequate drainage)
• •Asbestos or contamination found on opening up
• •Ground conditions worse than anticipated (softer bearing, buried rubble, proximity of services to new foundations)
• •Party wall damage discovered during works requiring immediate making good

• *Category 2 — Design development*:
• •Details that were not fully resolved at estimate stage and are clarified during construction in a way that adds cost
• •Client decision to upgrade a specification element (flooring, windows, kitchen) after the contract is signed — though this is more correctly a variation than a contingency draw-down
• •Building Control requirements identified during inspections that were not anticipated in the estimate (e.g., a requirement for additional fire-stopping or acoustic treatment)

• *Category 3 — Programme and inflation risk*:
• •Extended programme due to unforeseen conditions (a 12-week extension programme that runs to 16 weeks because of ground conditions)
• •Material price increases during a long project (relevant for projects over 6 months in length)
• •Procurement delays requiring alternative materials at higher cost

• **What contingency does NOT cover**:
• •Scope changes instructed by the client that are discretionary additions (a new bathroom added after contract; a change from standard to heated flooring) — these are variations and should be priced, agreed, and instructed in writing as additional to the original contract
• •Poor workmanship by the contractor that requires remediation — this is the contractor's risk and should be rectified at their cost under the defects liability provisions of the contract
• •Price increases resulting from the client failing to make decisions on time (the contractor claims extra time and cost because the client delayed approving tile choices for three weeks) — programme management is the client's and project manager's responsibility
• •The builder's preliminary and overhead costs in an extended project — the contract should be clear about whether any programme extension triggers additional preliminaries, and this should be managed by the project manager, not absorbed by contingency

**How to manage contingency correctly**:

1. *Establish the contingency as a named, separate budget item from the outset* — it must not be hidden within the contractor's price or assumed to be covered by the contract sum 2. *All drawdowns from contingency must be approved in writing* — every contingency spend must be evidenced by a specific unforeseen event or condition 3. *Review the contingency balance at each major stage milestone* — foundation, first-fix, second-fix. At each stage, assess whether the remaining contingency is proportionate to the remaining risk 4. *Do not treat unspent contingency as a saving until the defects liability period has expired* — defects identified after practical completion but within the 6–12 month defects liability period may require expenditure from the retained contingency 5. *For professional fees contingency*: allow a separate 5–10% contingency on professional fees — architects and structural engineers often need additional design time for unforeseen conditions that are not within their original fee scope

**Contingency vs. provisional sums**: A provisional sum is a fixed amount included in the contract for a specific item of work that cannot be fully defined at tender stage (e.g., provisional sum for specialist drainage works: £3,500). When the item is executed, the actual cost is substituted for the provisional sum. A contingency is an unallocated reserve for genuinely unforeseen events. Both are needed in any well-managed renovation project — they are not the same thing.

**Example 1: Victorian terrace rear extension**: Original contract sum: £75,000 (including client fee, contractor works, professional fees) Contingency held: 15% = £11,250

• *Contingency events encountered*:
• •Discovery of a filled Victorian well under the foundation line: additional foundation design and alternative foundation strategy required — additional cost: £4,800
• •Chimney breast on ground floor found to have no adequate support for the chimney breast above on the first floor (removed without support at an earlier date by previous owner): remedial steelwork to support the remaining chimney breast — additional cost: £2,100
• •Lead supply pipe from road to kitchen: replacement with blue MDPE from stop-cock to boiler — additional cost: £2,300
• •Total contingency used: £9,200
• •Contingency unspent (retained for defects period): £2,050

**Example 2: Full house refurbishment, unlived-in property**: Original contract sum: £140,000 Contingency held: 20% = £28,000

• *Contingency events encountered*:
• •Partial rewire found insufficient — full rewire required throughout: additional cost: £6,500
• •Rubber-insulated wiring in ceiling voids: licensed removal and replanning of cable routes: additional cost: £900
• •Ground floor concrete slab found to be unbonded and cracked: full break-out and new 100mm concrete slab with insulation — additional cost: £8,200
• •Artex on bedroom ceilings tested positive for chrysotile asbestos: licensed removal before skim coat: additional cost: £3,400
• •Total contingency used: £19,000
• •Contingency unspent (retained for defects period): £9,000

In both examples, contingency was used for genuine unforeseen conditions discovered on opening up — not for scope additions or estimating errors. The 10% rule would have left the first example project adequately covered but would have been insufficient for the second by £9,000.