First floor Extensions
Yes, several of these systems can be adapted to extend over an existing 15-year-old traditional extension, but success depends entirely on the structural capacity of your current foundations and walls.
Feasibility & Structural Requirements
The Critical Constraint: Foundations
A 15-year-old traditional extension likely has standard strip foundations designed only for a single storey. Adding a second floor (even a lightweight prefab one) significantly increases the load.
- Structural Assessment is Mandatory: You must hire a structural engineer to dig trial holes to inspect the depth and width of your existing footings.
- Underpinning: If the existing foundations are insufficient (which is common for single-storey builds), they will need underpinning or reinforcement before any prefab system can be installed on top. check foundations spec soon
Best Systems for “Building Up”
Lightweight construction methods are essential for minimizing the load on your existing structure.
Steel Frame Systems (Top Recommendation)
Light Gauge Steel Frame (LGSF) systems are the most suitable option for building over existing extensions due to their high strength-to-weight ratio.
- Why they work: They are significantly lighter than timber or brick, reducing the stress on existing walls and foundations.
- Suppliers: Companies like U-Roof and Steel Frame Buildings Ltd specialize in lightweight steel kits that can be craned into position. Their panels are often thin yet structurally robust, maximizing internal space without adding excessive weight.
SIPs (Structural Insulated Panels)
Systems like those from Extension Kitz or Siphouse use SIPs, which are lighter than traditional blockwork but heavier than thin-gauge steel.
- Suitability: Viable if your existing foundation is robust or requires only minor reinforcement.
- Benefit: They offer superior insulation and speed of assembly, often being installed in days once the structural prep is complete.
Warmer Room & Timber Modules
While Warmer Room kits are lightweight compared to brick, a full timber module still carries significant weight.
- Application: These are better suited if your existing extension was “over-engineered” (built with double-storey foundations in mind) or if you are willing to invest in underpinning first.
- Hybrid Approach: Some homeowners use these systems for the walls but opt for a lighter roof structure to reduce the overall load.
Key Considerations for Your Project
1. Movement & Ties
Connecting a new prefab structure to a 15-year-old building requires accommodating thermal movement. The two structures will expand and contract at different rates.
- Solution: Suppliers typically use stainless steel channel ties or movement gaps to prevent cracking where the new module meets the old roof/walls.
2. Access & Craneage (Modular ONLY)
Installing a prefab unit over an existing roof often requires a large crane.
- Constraint: Ensure you have adequate access for a crane to lift the module over your existing house and lower it precisely onto the extension. If crane access is impossible, panelized systems (like SIPs or Steel Frames) that are assembled piece-by-piece on the roof are the alternative.
3. Cost Implications
While the prefab unit itself saves time, the preparatory work for a second storey is costly.
- Estimated Costs: Expect to pay £1,800–£2,500 per m² for the complete project, including structural reinforcements, crane hire, and the kit. If underpinning is required, this cost will rise significantly.
Specifics
i would only consider steel or SIP panels and NOT “modular” homes that need craneage. are you able to estimate costs for 50 sqm first floor extension which would then include a roof and also habitable loft Yes, both Steel Frame and SIP (Structural Insulated Panel) systems are ideal for your requirements as they are lightweight, panel-based solutions that do not require large modular cranes. They are assembled piece-by-piece on-site, often using smaller lifting equipment or manual handling.
Here is the estimated cost breakdown for a 50 sqm first-floor extension (including the new roof and habitable loft space) in 2026.
Estimated Total Project Cost: £95,000 – £145,000+
(Excluding VAT and potential underpinning costs)
This estimate assumes a “turnkey” finish (plastered, decorated, basic kitchen/bathroom fit-out) but excludes high-end bespoke joinery or luxury flooring.
1. Construction Costs (Panel System & Build)
The cost per square metre for a two-storey extension (which your project effectively is, as you are building a new floor and roof) ranges based on the system and finish quality.
- Light Gauge Steel Frame (LGSF): £1,900 – £2,600 per m².
- Why: Slightly higher material cost but extremely fast erection. Ideal for complex shapes.
- Subtotal: £95,000 – £130,000
- SIPs (Structural Insulated Panels): £1,400 – £2,200 per m² (panel kit + erection).
- Why: Often cheaper on the shell, offers superior airtightness immediately.
- Subtotal: £70,000 – £110,000
Note: The lower end of these ranges typically covers the “shell and weatherproof” stage. The upper end includes full internal fit-out (electrics, plumbing, plaster, flooring).
2. The “Hidden” Critical Cost: Structural Reinforcement
Since you are building over a 15-year-old traditional extension, this is the most variable cost factor.
- Structural Engineer Survey: £600 – £1,400.
- Mandatory to dig trial holes and assess existing footings.
- Underpinning (If required): £15,000 – £30,000+.
- Most single-storey extensions from 15 years ago were not built with foundations deep enough to support a second storey.
- Cost is approx. £1,500 – £2,000 per m² of underpinning required. If the whole perimeter needs work, costs rise sharply.
- If your existing foundations pass: You save this entire amount.
3. Roof & Habitable Loft Specifics
Creating a “habitable loft” within this extension involves specific costs distinct from a standard flat roof:
- Roof Structure: Included in the m² rates above if using a pitched SIPs/Steel roof system.
- Windows & Glazing: £8,000 – £15,000.
- Habitable rooms require significant natural light (typically 20% of floor area). Large roof windows (e.g., Velux) or dormer glazing are expensive but essential for compliance.
- Staircase: £2,500 – £6,000.
- A new fixed staircase to access the habitable loft is required. Space planning on the existing first floor is critical here.
Summary Cost Table (50 sqm)
| Item | Low Estimate (SIPs, No Underpinning) | High Estimate (Steel, Full Underpinning) |
|---|---|---|
| Panel System & Build | £70,000 | £130,000 |
| Roof Windows & Glazing | £8,000 | £15,000 |
| Staircase Installation | £2,500 | £6,000 |
| Professional Fees | £5,000 | £12,000 |
| Underpinning (If needed) | £0 | £30,000+ |
| Total (excl. VAT) | ~£85,500 | ~£193,000+ |
Key Takeaways for Your Strategy
- Start with the Survey: Do not design anything until a structural engineer confirms if your existing foundations can take the load. This single fact determines if your budget is £90k or £140k+.
- SIPs vs. Steel: For a 50 sqm area, SIPs might offer a slight cost advantage on the shell and superior thermal performance out of the box. Steel offers more flexibility if your design has large open spans or complex junctions with the existing house.
- No Crane Needed: Both systems use panels that can be lifted by 2-4 people or a small telehandler, avoiding the need for the large cranes required for “modular pod” systems.
Top Modular & Prefab Extension Suppliers
Here are the leading prefab extension suppliers in the UK, categorized by their specific construction methods and market positioning. These companies offer alternatives to traditional building, significantly reducing on-site construction time and mess.
Warmer Room (Self-Build & Trade Kits)
Warmer Room specializes in self-build extension kits designed for both homeowners and trade professionals. Their system focuses on high thermal performance and speed, with most kits installable in just 5 days. The kits arrive as pre-assembled modular pods featuring 160mm structural timber walls and premium insulation, ensuring a “cool in summer, warm in winter” environment. They offer a 25-year guarantee and handle everything from the roof system (pitched or flat) to insulated plasterboard, requiring minimal excavation.
EazyBase (Affordable Full-Service)
EazyBase positions itself as an affordable provider of prefabricated modular home extensions available across the UK. They manage the entire process from design to installation, building the extension in their factory within 4 weeks and assembling it on-site in less than a week. Their service is comprehensive, including foundations, brickwork, electrics, plastering, and flooring. They are particularly popular for creating home offices, playrooms, and kitchen extensions, offering a “one-stop-shop” solution that eliminates the need for coordinating multiple trades.
Durabase (Steel Base & Real Brick)
Durabase offers a unique system combining prefabricated steel frames with real brick modular walls. With over 40 years of experience, their method requires only concrete pads or ground screws for foundations, drastically reducing excavation waste. The extensions are manufactured off-site in Hereford and delivered in kit form for rapid assembly. This system is suitable for both professional installation and confident DIYers, providing the aesthetic of a traditional brick extension with the speed of modular construction.
HUF HAUS (Premium German Prefab)
For those seeking a high-end, architectural solution, HUF HAUS brings German engineering to the UK market. They specialize in modern prefab extensions, including significant two-storey additions and roof extensions that can add over 200 square meters of space. Their designs often feature floor-to-ceiling glass and modern half-timbered architecture, complying with rigorous efficiency standards (Efficiency House 55). This option is ideal for homeowners looking for a seamless, premium integration with existing properties rather than a standard kit.
Ultraframe (Roof Systems & Conservatory Upgrades)
Ultraframe is a primary supplier of prefabricated roof systems and modular extensions, often installed through their network of approved “Ultra Installers.” They are well-known for the Livinroof and Ultraroof systems, which are ideal for replacing conservatory roofs or creating new solid-roof extensions. Their systems are fully compliant with building regulations, thermally efficient, and customizable with lighting and speakers. Unlike flat-pack online options, Ultraframe provides a certified installation process with warranties and building control liaison.
Siphouse (Timber Frame Panels)
Siphouse (Sipland) focuses on modular timber frame panels that allow for rapid expansion without traditional bricklaying. Their approach is highly customizable, enabling homeowners to design extensions that match their current home’s aesthetic perfectly. They offer bespoke services to tailor dimensions and designs, positioning their timber frame solutions as a cost-effective alternative to moving house. Their systems are noted for being thermally efficient and quick to erect on prepared footings.
Key Benefits of Prefab Extensions
- Speed: Most systems reduce on-site build time from months to days or weeks.
- Reduced Mess: Off-site manufacturing minimizes excavation, waste, and on-site disruption.
- Thermal Efficiency: Modern prefab units often exceed traditional builds in insulation and air tightness.
- Cost Certainty: Factory production allows for fixed pricing, reducing the risk of budget overruns common in traditional builds.