RSJ Beam for Kitchen Extension – Real Costs and Sizing Examples in 2026
Kitchen extensions are among the most popular home improvements, transforming cramped kitchens into spacious kitchen-diners or open-plan living spaces. RSJ steel beams play crucial roles in these projects, supporting walls, floors above, and creating the large openings that make extensions so appealing. This guide provides real-world examples, accurate costs, and practical sizing guidance for kitchen extension projects in 2026.
Common Kitchen Extension Configurations
Single-Story Rear Extension
Typical dimensions:
- 3-4m depth extension
- Full width of property (3-6m)
- Flat or pitched roof
- Bi-fold or sliding doors to garden
RSJ requirements:
- Rear wall beam (spans width of extension where connects to house)
- Side wall beams (if removing existing external walls)
- Roof support (if flat roof with parapet walls)
Two-Story Extension
Typical dimensions:
- 3-4m depth
- Full width
- Bedroom/bathroom above kitchen
- More complex loading
RSJ requirements:
- Ground floor rear beam (heavier loads from floor above)
- First floor front beam (for bedroom opening)
- Multiple supporting beams for floor structure
Side Return Extension
Common in terraced houses:
- Infill narrow side passage
- Usually 1.5-2.5m wide
- Connects kitchen to side space
- Creates L-shaped kitchen-diner
RSJ requirements:
- Side wall beam (where existing wall removed)
- Front connection beam (if opening up to hallway)
Beam Sizing Examples
Example 1: Simple Single-Story Extension (3.5m wide)
Configuration:
- Semi-detached house, 3.5m wide
- 3m deep single-story extension
- Flat roof
- No room above (roof load only)
Critical beam: Rear wall connection
- Span: 3.8m (3.5m opening + bearing)
- Loads:
- Existing external wall removed: 8 kN/m
- Extension roof (flat, felt): 2.5 kN/m per side × 2 = 5 kN/m
- Total: ~13 kN/m
Beam selection: 203×133×30 RSJ
- Capacity at 3.8m: ~14 kN/m ✓
- Cost: £320-410
Additional beams:
- Possibly 152×127×37 for roof structure support
- Cost: £190-260 (2.5m)
Total beam cost: £510-670
Example 2: Single-Story Extension with Bi-Folds (5m wide)
Configuration:
- Detached house, 5m wide
- 4m deep extension
- 4m wide bi-fold doors to garden
- Pitched roof
Critical beam: Rear wall + bi-fold header
- Span: 5.3m
- Loads:
- Full brick wall above (up to eaves): 11 kN/m
- Roof loads: 7 kN/m
- Total: ~18 kN/m
Beam selection: 305×165×40 UB
- Capacity at 5.3m: ~19 kN/m ✓
- Cost: £525-680 (5.6m length)
- Heavy beam - mechanical lifting required
Side wall beams (if needed):
- 254×146×31 × 2 for side walls
- Cost: £240-320 each
Total beam cost: £1,005-1,320
Example 3: Two-Story Extension (4m wide)
Configuration:
- Terraced house, 4m wide
- 3.5m deep extension
- Kitchen below, bedroom above
- Bathroom in new bedroom space
Ground floor rear beam:
- Span: 4.3m
- Loads:
- Brickwork: 9 kN/m
- Bedroom floor structure: 8.5 kN/m
- Bathroom loads (bath, fixtures): additional 2.5 kN/m
- Total: ~20 kN/m
Beam selection: 254×146×37 RSJ
- Capacity: ~21 kN/m at 4.3m ✓
- Cost: £390-510
First floor bedroom front beam:
- Span: 4.3m
- Loads: Wall + roof = 12 kN/m
- Beam: 254×146×31 RSJ
- Cost: £330-430
Floor support beams (internal):
- 203×133×25 × 2 for spanning to external walls
- Cost: £260-340 each
Total beam cost: £1,240-2,020
Example 4: Side Return Extension (2m wide)
Configuration:
- Victorian terraced house
- 2m wide side return
- 4.5m length
- Single story with roof light
Side wall beam:
- Span: 4.8m (length of side return)
- Loads:
- Party wall or flank wall: 7.5 kN/m
- Roof structure: 4 kN/m
- Total: ~11.5 kN/m
Beam selection: 254×146×31 RSJ
- Capacity: ~14 kN/m at 4.8m ✓
- Cost: £350-460
Connection to house beam:
- 203×133×25 (2.5m)
- Cost: £185-245
Total beam cost: £535-705
Complete Cost Breakdowns
Single-Story Extension (3.5m × 3m)
| Item | Cost Range |
|---|---|
| Design & Approval | |
| Architect/designer | £800-1,500 |
| Structural engineer | £500-900 |
| Building Regs fee | £400-650 |
| Groundworks | |
| Excavation & disposal | £800-1,400 |
| Foundations (strip/trench) | £1,800-3,200 |
| Ground slab & DPM | £800-1,300 |
| Structure | |
| RSJ beams | £510-670 |
| Blockwork/brickwork | £2,400-4,200 |
| Roof structure (flat) | £1,600-2,800 |
| Windows & Doors | |
| Bi-fold doors (3m) | £1,800-4,500 |
| Roof light | £400-900 |
| Services | |
| Electrics | £800-1,500 |
| Plumbing | £600-1,200 |
| Heating | £700-1,400 |
| Finishes | |
| Plastering | £900-1,600 |
| Flooring | £600-1,200 |
| Kitchen units/worktops | £3,000-8,000 |
| Decoration | £500-1,000 |
| Total | £19,510-37,820 |
Professional build: £25,000-38,000 With significant DIY: £20,000-28,000
Two-Story Extension (4m × 3.5m)
| Item | Cost Range |
|---|---|
| Design & Approval | |
| Architect | £1,200-2,200 |
| Structural engineer | £700-1,300 |
| Building Regs | £500-750 |
| Ground works | |
| Foundations (deeper for 2-story) | £2,800-4,800 |
| Ground slab | £900-1,500 |
| Structure | |
| RSJ beams (multiple) | £1,240-2,020 |
| Blockwork (2-story) | £4,800-7,500 |
| First floor structure | £2,200-3,800 |
| Roof structure (pitched) | £2,500-4,200 |
| Windows & Doors | |
| Ground floor bi-folds | £2,200-4,800 |
| First floor windows | £800-1,600 |
| Services | |
| Electrics (2 floors) | £1,400-2,400 |
| Plumbing (bathroom) | £1,800-3,500 |
| Heating (2 floors) | £1,200-2,200 |
| Finishes | |
| Plastering (2 floors) | £1,800-3,000 |
| Bathroom fitout | £3,000-7,000 |
| Kitchen | £3,500-9,000 |
| Flooring (2 floors) | £1,000-2,000 |
| Stairs (if needed) | £800-2,000 |
| Decoration | £900-1,800 |
| Total | £35,440-63,870 |
Professional build: £40,000-65,000 With DIY finishing: £35,000-52,000
Design Tips for Kitchen Extensions
Maximizing Natural Light
Roof lights:
- Minimum 20% of floor area for good daylighting
- Flat glass roof panels increasingly popular
- Lantern roof lights for period properties
- Cost: £400-1,200 per roof light
Bi-fold/sliding doors:
- Full-width doors maximize light and garden connection
- Bi-folds: £600-1,500 per meter width
- Slim-frame sliding doors: £800-2,000 per meter
Side windows:
- Frosted glass if privacy needed
- High-level windows for light without overlooking
- Internal glazing to adjacent rooms
Internal Layout
Kitchen work triangle:
- Sink, hob, fridge should form efficient triangle
- Maximum 6m total perimeter
- No traffic cutting through triangle
Island units:
- Minimum 1m clearance all sides
- Consider ceiling beams - may restrict island size
- Plumbing/electrical planning for island
Storage:
- Floor-to-ceiling units maximize space
- Pantry cupboards very popular
- Drawer banks more accessible than deep cupboards
Hiding the Beam
Box in with plasterboard:
- Standard approach
- Paint to match ceiling
- Can incorporate spotlights
- Cost: £80-150 per meter
Feature beam:
- Leave exposed, paint contrasting color
- Industrial/contemporary aesthetic
- Must be straight and well-finished beam
Bulkhead:
- Lower full width of ceiling
- Conceals beam and services (extract ducts etc.)
- Creates defined zones
- Cost: £200-400 for bulkhead across 4m
Building Regulations Considerations
Planning Permission
Usually NOT required if:
- Single-story extension
- <4m deep (or <3m if terraced/semi)
- <4m high
- <50% of garden area
- Not forward of principal elevation
Usually REQUIRED for:
- Two-story extension
- Side extension >50% property width
- In conservation area or listed building
- Exceeds permitted development limits
Check before proceeding! Fee: £206-462
Building Regulations (Always Required)
Structural (Part A):
- Beam calculations essential
- Foundation design
- Wall ties, DPCs
Fire Safety (Part B) if two-story:
- Escape routes
- Fire doors
- Compartmentation
Sound Insulation (Part E):
- Floor between kitchen/bedroom above
- Party walls
Drainage (Part H):
- New waste pipes
- Surface water drainage
Energy Efficiency (Part L):
- U-values: Walls 0.18, Roof 0.15, Floor 0.15 (2024 regs)
- High-performance glazing
- Continuity of insulation
Fee: £400-750 depending on project value
Common Problems
Problem 1: Underestimating beam sizes
Many DIYers use rules of thumb and end up with inadequate beams.
Example error: “My neighbor used 203×133×25 for 3m so I’ll use same for my 4.5m span”
Result: Excessive deflection, bouncy floor, possible failure
Solution: ALWAYS use structural engineer for beam sizing
Problem 2: Ignoring drainage
Extensions create new roof area = more rainwater
Issues:
- Inadequate gulley size
- No soakaway
- Surface water flooding
Solution: Design drainage properly from start, not afterthought
Cost to fix later: £800-1,500
Problem 3: Poor ventilation
Combining kitchen with dining/living increases moisture/cooking smells
Issues:
- Condensation
- Lingering odors
- Mold growth
Solutions:
- Enhanced extract fan (minimum 60 l/s)
- Background ventilation (trickle vents)
- Consider mechanical ventilation with heat recovery (MVHR)
Problem 4: Not future-proofing electrical
Extensions often lead to more kitchen appliances
Common regrets:
- Not enough sockets (minimum 6 double sockets in kitchen)
- No provision for future island appliances
- Inadequate circuit capacity
Solution: Over-spec electrics - adding later is expensive
Problem 5: Forgetting about access during build
Large beams don’t fit through doors!
Solutions identified in planning:
- Crane lift over house
- Pass through removed window
- Cut temporary opening in wall
Cost if not planned: £300-800 emergency crane hire
Professional vs. DIY
Realistically, kitchen extensions require professionals for:
- Structural engineering (essential)
- Foundations and groundworks
- Beam installation
- Building Control liaison
- Gas work (if applicable)
- Electrical work (part P compliance)
Possible DIY elements:
- Internal finishing
- Tiling
- Kitchen unit installation (if competent)
- Decoration
- Landscaping after
Realistic savings from DIY finishing: £3,000-6,000
DON’T attempt to DIY:
- Foundation design or excavation
- Structural beam installation
- Any work requiring Building Regs sign-off without qualifications
Return on Investment
Value added vs. cost:
Single-story extension:
- Cost: £25,000-38,000
- Value added: £30,000-50,000
- ROI: 120-130%
- Recoups 100% of cost typically
Two-story extension:
- Cost: £40,000-65,000
- Value added: £50,000-80,000
- ROI: 120-125%
- Excellent value for money
Kitchen extensions among best ROI home improvements
Additional benefits:
- Improved livability (better than financial return)
- Lifestyle enhancement (worth more than £)
- Prevents need to move (saves £10,000+ in moving costs)
Conclusion
Kitchen extensions using RSJ steel beams are excellent home improvements offering strong ROI and lifestyle benefits. Proper beam sizing through structural engineering, realistic budgeting, and Building Regulations compliance ensure successful projects.
**Key
planning points:**
- Engage professionals early - architect + structural engineer
- Budget realistically - single story £25k-38k, two-story £40k-65k
- Plan beam access - large beams are heavy and awkward
- Don’t undersize beams - false economy leads to problems
- Over-spec services - cheaper now than retrofitting later
- Consider future - design for potential further changes
Typical beam costs: £500-2,000 depending on project complexity = small fraction of total project cost but critical for success.
Disclaimer: Costs are indicative 2026 UK estimates and vary by location and specification. All kitchen extension projects require structural engineering design and Building Regulations approval. This guide provides general information only.
