Loft Conversion & Basement, Surrey

A homeowner in Guildford, Surrey wanted to maximise the living space in their detached family home by converting the loft into a master bedroom suite and excavating a basement to create a home cinema and utility room. The combined project would add significant floor area without extending the building’s footprint.

The client needed phased estimates to manage their cash flow effectively. The plan was to complete the loft conversion first, then begin the basement excavation approximately six months later. Each phase needed to be independently viable with its own detailed cost breakdown.

Combining two major structural projects under one estimate while keeping them independently viable presented a significant estimating challenge. Shared elements such as preliminaries, scaffolding and temporary works needed careful allocation between the two phases to ensure each stood alone as an accurate budget.

The basement excavation near existing foundations required careful engineering, with underpinning costs depending on the depth and proximity to load-bearing walls. Additionally, the property sat within a conservation area, which imposed specific material and design requirements that added both specification complexity and cost.

• Two separate detailed estimates with a shared preliminaries analysis, ensuring each phase could proceed independently or as a combined project
• Structural steelwork and underpinning cost breakdown, with detailed quantities for the basement phase based on the structural engineer’s design
• Waterproofing specification and tanking system comparison, evaluating three different systems with cost and performance analysis
• Phased programme with cost-flow projection, showing monthly expenditure forecasts for both phases to support the client’s financial planning
• Conservation area compliance cost additions, identifying the premium associated with approved materials, design detailing and planning requirements

The phased approach allowed the client to complete the loft conversion first and use the resulting equity gain to help fund the basement phase. By value engineering the tanking system — switching from a cavity drain membrane to a more cost-effective cementitious waterproofing solution on our recommendation — the total project came in at 5% under the combined estimate. The full programme was completed in 14 months.