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Extensions

Two-storey extension estimate.

Two-storey extensions add significant floor area but bring extra complexity — structural steel, taller scaffolding, and coordinating finishes across two floors. This guide helps builders price the additional work accurately.

Authored by Alexandr Vreme & Maksym Vasylkov
Published July 2026 — Next review July 2027

Who this guide is for

For builders and estimators pricing two-storey rear or side extensions. The extra storey adds structural, access, and programme complexity that single-storey experience alone does not fully prepare you for.

Information you need before pricing

In addition to the information required for a single-storey estimate, you will need:

• Staircase location and headroom — can a straight flight fit, or do you need a half-landing?
• First-floor joist spans — engineered joists or Posi-Struts may be needed for clear spans
• Steel transfer beams — typically 2–4 beams to support the first floor and roof loads
• Roof tile matching — finding like-for-like tiles for the extension roof
• Existing foundation capacity — deeper strip or piled foundations to carry additional storey loads

Extra items over a single-storey extension

• Structural engineer design and calculations for steelwork and foundations
• Steelwork — typically 2–4 steel beams with padstones and fire protection
• First-floor joists or engineered cassettes
• Staircase — supply and install, plus making good the ceiling below and any half-landing
• Additional first-floor plumbing and electrics
• Extra scaffolding height and loading for brick and materials at first-floor level
• Longer programme — typically 16–24 weeks versus 8–12 for single-storey

Typical cost breakdown

As a rough guide, the cost splits roughly:

• Ground floor works: ~55% of total cost
• First floor works: ~35% of total cost
• Roof works: ~10% of total cost

Commonly missed costs

• Temporary propping during steel installation — acrow props, needle beams, propping design
• Scaffold design certification for two-storey height
• Material hoisting — cranage or telehandler for steel beams and floor cassettes
• Longer prelims — extended site welfare, supervision, and hire costs over a 16–24 week programme
• Party wall awards for deeper foundations and steel beam bearing on shared walls

Miniature worked example

A 30 m² footprint two-storey rear extension with pitched roof:

• Ground floor (foundations, slab, walls, doors, finishes): £48,000
• First floor (steel, joists, walls, staircase, finishes): £38,000
• Preliminaries (scaffold, skip, welfare, supervision, design): £14,000

Total: approximately £100,000. For comparison, a single-storey extension of the same 30 m² footprint would be around £75,000. The two-storey premium reflects steelwork, staircase, and additional finishes.

Suitable output documents

• Detailed estimate with preliminaries separated from direct works
• Priced bill of quantities showing ground and first floor separately
• Programme of works with staged payment milestones

Questions to ask before pricing

• Has a structural engineer produced steel calculations yet?
• Where will the staircase land, and does it need a half-landing?
• What existing roof covering is on the house — can it be matched?
• Is there room for cranage or a telehandler on site?