Planning an extension is exciting. But you need to understand building regulations drawings and planning permission first. This guide covers building a single storey extension or two-storey addition. Following the building regulations keeps your house extension safe, energy-efficient, and legal. We explain when you need planning permission for a single storey extension, what building regulations apply, and how to get approval for any storey extension or alteration.
What are the Key Building Regulations for Extensions?
Building regulations set minimum standards for construction and alteration of buildings in England. Every homeowner planning a house extension must comply with the building regulations. This applies whether or not you need planning permission. These rules cover structural strength, fire safety, and more. They make sure your building project meets required standards.
Foundations
Your foundation design must account for ground conditions, soil type, and weight loads. Building Control needs calculations showing your foundations prevent settlement or movement. Strip foundations work for most extensions. You might need raft or trench-fill foundations depending on your site.
The design must meet Approved Document A (Structure). Pay attention to existing foundations. Avoid undermining adjacent buildings. Ground conditions vary across London—from London Clay to made ground. You’ll need site-specific assessments. Your building contractors arrange foundation inspections before pouring concrete.
Ground Floors
Ground floors must meet thermal standards under Approved Document L. You’ll need insulation beneath the floor. This achieves U-values of 0.22 W/m²K or better. You can use suspended timber floors, solid concrete slabs, or beam-and-block systems. They must comply with structural and thermal requirements.
Damp-proofing is critical. Ground floors need a damp-proof membrane. This stops moisture getting in. It connects to the damp-proof course in walls. Ventilation beneath suspended timber floors prevents rot. It keeps your storey extension project structurally sound.
Main Walls
External walls must meet several requirements. These include structural stability (Part A), fire resistance (Part B), moisture resistance (Part C), and thermal performance (Part L). Cavity wall construction with insulation is standard. This achieves U-values around 0.18 W/m²K. It helps comply with the building regulations.
You’ll need wall ties, damp-proof courses, and proper detailing. This matters where new work joins existing structures. Extensions touching neighbouring properties need Party Wall Act compliance. You need written permission before work starts. The new extension must tie into the rear wall of the original house.
Structural Openings
Any opening in load-bearing walls needs proper support. This includes doors, windows, or connections to the existing building. Support typically uses steel beams or reinforced concrete lintels. Structural calculations must show adequate load transfer.
Removing internal walls creates open-plan spaces. This is common when building an extension. You must get Building Control approval first. Never remove load-bearing elements without it. Calculations specify beam sizes, bearing lengths, and temporary support. This alteration is critical for building regulations approval.
Upper Floors
Two-storey extensions need upper floors meeting structural requirements. These control deflection and vibration. They provide fire resistance between floors. Timber joist floors or engineered timber I-joists are typical. Structural design calculations are required for every storey.
Sound insulation between floors must meet Approved Document E. This matters for loft conversions or extensions creating living rooms above existing spaces. You may need acoustic testing. This demonstrates compliance with building regs.
Roofs
Roof construction must meet structural, thermal, and fire safety requirements. Pitched roofs typically use timber trusses or traditional cut roofing. They handle loads including snow and wind. Flat roofs need careful design. They require adequate drainage slopes and weatherproofing.
Thermal performance is critical. Roof insulation must achieve U-values of 0.15 W/m²K or better. This complies with building regulations. Extensions close to boundaries may need upgraded fire resistance. This applies within one metre of the boundary. Special measures apply under Approved Document B.
Chimney Stacks and Flues
New heating appliances or existing chimneys must meet Approved Document J requirements. This covers combustion appliances and fuel storage. Flue design, terminal positions, and ventilation depend on appliance type. Gas boilers, wood-burning stoves, and oil-fired systems each have specific rules.
Extensions affecting existing chimneys need assessment. Check their structural condition. Determine if they can stay or need removing. Provide proper support as part of the building project.
Windows and Doors
Windows and external doors must meet thermal standards (Part L). Typical U-values are 1.4 W/m²K for windows and 1.0 W/m²K for doors. Glazing must meet safety requirements (Part N). Use toughened or laminated glass in critical locations. This includes low-level glazing or doors.
Ventilation requirements under Approved Document F set minimum openable areas. Habitable rooms typically need 1/20th of floor area for background ventilation. Plus purge ventilation via opening windows. Emergency escape from first-floor bedrooms affects window sizes. It also affects sill heights in your house extension.
Internal Walls
Internal partition walls don’t need to be load-bearing. But they must provide adequate fire resistance. This matters for protected escape routes. Sound insulation requirements apply to walls separating living rooms from bathrooms or WCs. This applies throughout your home extension.
Internal walls supporting upper floors or roofs need structural design. Stud walls, blockwork, or lightweight steel frame systems all work. Choice depends on loading and acoustic requirements. This varies for each part of the building.
Internal Joinery
Staircases serving extensions must meet dimensional requirements. Maximum pitch is 42 degrees. Minimum headroom is 2m. Riser heights must be consistent (typically 220mm maximum). Minimum going depths are typically 220mm. Handrails are required where flights exceed two risers. Position them between 900-1000mm above the pitch line.
Guards to balconies, landings, or level changes must prevent falls. Typically 1100mm high for areas not readily accessible to children. Design them to prevent climbing. These are essential building regulations requirements for any storey extension.
Services
Electrical installations must meet BS 7671 (18th Edition Wiring Regulations). Notify Building Control or get certification by a registered competent person scheme. New circuits, consumer unit upgrades, and wet area installations need particular attention when building an extension.
Plumbing installations must meet water efficiency requirements (Part G). WCs are limited to 6-litre dual-flush maximum. Taps and showers have flow restrictions. Hot water systems need safety controls preventing scalding.
Heating systems must meet energy efficiency standards. Proper controls include thermostatic radiator valves, room thermostats, and time controls. Ventilation for combustion appliances and mechanical ventilation for wet rooms ensure safe environments. This creates healthy conditions in your new extension.
We prepare comprehensive building regulation drawings for extensions, conversions, and alterations. Detailed plans with structural calculations that satisfy Building Control and keep your project on track.
Learn more about building regulation drawings →Planning Permission for Extensions: What You Need to Know
Do I Need Planning Permission for a Single Storey Extension?
Many homeowners ask this question when planning an extension. The answer depends on size, location, and design of your proposed house extension. Permitted development rights may let you build a single storey extension without planning permission. It must meet specific size limits and conditions.
Single storey rear extensions up to 6 metres deep for semi-detached or terraced houses don’t usually need planning consent. 8 metres applies for detached houses. However, permitted development rights have strict criteria. You must meet these to build your extension without planning approval.
Building a Single Storey Extension Without Planning Permission
To build a storey extension without planning permission, your proposal must meet several requirements:
- The extension must not exceed 4 metres in height
- Eaves must not exceed 3 metres in height
- The extension must not extend beyond the principal elevation facing a highway
- The extension must not exceed half the area of land around the original house
- Materials must look similar to the existing house
Single storey rear extensions exceeding 3 metres (attached houses) or 4 metres (detached houses) need prior approval. This is a streamlined process. Neighbours are consulted but full planning permission isn’t required.
When You Must Apply for Planning Permission
You’ll definitely need to apply for planning permission in these cases:
- Listed building extensions or alterations always require listed building consent
- Properties in conservation areas have restricted permitted development rights
- Areas with Article 4 directions remove permitted development rights
- Side extensions wider than half the width of the original house
- Extensions forward of the principal elevation
- Two-storey extensions exceeding permitted development limits
- Properties where permitted development rights were removed by planning conditions
Lawful Development Certificate for Extensions
You may believe you can build a single storey extension without planning permission. Getting a lawful development certificate provides valuable confirmation. This certificate proves your extension benefits from permitted development rights. You don’t need full planning consent.
A lawful development certificate protects you when selling your property. Buyers’ solicitors often query extensions built without formal planning approval. The certificate shows your extension was lawful. This avoids problems during conveyancing.
Single Storey Rear Extensions: Planning Considerations
Single storey rear extensions are the most common house extension type. Many fall within permitted development rights. However, careful measurement is essential. Errors calculating how far your extension projects beyond the rear wall of the original house can result in enforcement action.
When planning a single storey rear extension, consider:
- Distance from boundaries (closer than 2 metres triggers height restrictions)
- Impact on neighbours’ light and outlook
- Drainage and access for maintenance
- Integration with the existing building’s design
Planning permission for a single storey extension isn’t always required. But good design respecting neighbouring properties reduces complaint risks. It prevents potential disputes.
Side Extension Planning Requirements
Side extensions face stricter planning controls than rear extensions. Under permitted development, a side extension must not exceed half the width of the original house. It must typically be single-storey only. Two-storey side extensions almost always need planning permission.
Many areas face additional local planning policies. This particularly applies in London boroughs. Side extensions policies prevent terracing effects. They preserve gaps between buildings. Always check local planning guidance when planning an extension to your property’s side.
Planning an Extension: Step-by-Step Process
Planning an extension requires careful preparation. This applies whether you need planning permission or use permitted development rights:
- Research permitted development rights – Find out if your proposal qualifies for exemption from planning permission
- Consider getting a lawful development certificate – Confirms your extension is lawful without formal planning consent
- Prepare detailed drawings – Essential even without planning permission, for building regulations approval
- Consult neighbours – Good practice even under permitted development
- Submit building regulations application – Required regardless of planning status
- Apply for planning permission if needed – Full application with supporting documents
Frequently Asked Questions
What are the building regulations for a single storey extension?
Single storey extensions must meet all applicable building regulations. These cover structural design, foundations, damp-proofing, thermal performance, fire safety, drainage, and electrical safety. Building Control approval is required regardless of extension size. Use either your Local Authority Building Control or an Approved Inspector.
Every homeowner must comply with the building regulations when building a single storey extension. Key considerations include U-values for walls (0.18 W/m²K), roofs (0.15 W/m²K), and floors (0.22 W/m²K). You need adequate structural support connecting to the existing building. Drainage connections must comply. Extensions within one metre of boundaries require upgraded fire resistance.
Some small extensions under 30m² may be exempt from building regulations. They must be at least one metre from any boundary. This exemption is rare in practice. Most single storey rear extensions require full building regulations approval. You must submit a building regulations application before starting work.
What size extension can I build without planning permission in 2025?
Permitted development rights in England allow single storey rear extensions up to 6 metres deep. This applies for a semi-detached or terraced house. 8 metres applies for a detached house. No planning permission needed. Two-storey extensions are allowed up to 3 metres beyond the rear wall of the original house.
You can build your extension without planning permission only when conditions are met. The single storey extension must not exceed 4 metres in height within 2 metres of the boundary. Extensions beyond the rear wall by more than 3 metres (attached houses) or 4 metres (detached houses) need prior approval. This is lighter-touch consent than full planning permission.
The extension must not exceed half the area of land around the original house. The width of the original house affects what you can build. Side extensions are more restricted. Development rights don’t apply in conservation areas or Areas of Outstanding Natural Beauty. They don’t apply for a listed building either. You typically need to apply for planning permission in these cases.
Remember: you must always comply with the building regulations even without planning permission. Planning consent and building regulations approval are separate processes. You need both to build legally.
Does the 45 degree rule apply to single storey extensions?
The 45-degree rule isn’t a building regulation. It’s a planning guideline. Many Local Planning Authorities use it to assess extensions. They check whether extensions cause unacceptable loss of light to neighbouring properties when you apply for planning permission. The test measures a 45-degree line from the centre of the nearest habitable room window in the neighbouring property.
Your proposed single storey extension breaching this line may indicate excessive impact. This applies when viewed in plan and elevation. It may affect neighbouring amenity. This can lead to planning refusal when planning an extension. However, it’s a guideline, not an absolute rule. Planning officers consider context, orientation, and site-specific circumstances.
Some boroughs have moved away from strict application of the 45-degree rule. Several London authorities favour more nuanced assessments. They use BRE guidelines for daylight and sunlight. This particularly applies for developments in dense urban areas. Understanding these planning permission for a single storey extension requirements helps homeowners plan effectively.
Do you need foundations for a single storey extension?
Yes, foundations are always required when building a single storey extension. Size doesn’t matter. Even modest one storey additions put significant loads on the ground. Building regulations require foundations preventing settlement, subsidence, or structural movement. These could damage the new extension or existing building.
Foundation design depends on several factors. These include ground conditions, proximity to trees, soil type, and structural loads. Strip foundations are typical for most single storey rear extensions. Depths of at least 1 metre in clay soils avoid seasonal movement. Poor or variable ground conditions may need deeper foundations. Alternative systems like piling may be necessary for your house extension.
Building Control inspects foundations before concrete is poured. They check correct dimensions and depth. They verify foundations don’t undermine adjacent structures. Never backfill foundation trenches before inspection. Doing so may require expensive remedial excavation. Your building contractors coordinate these inspections as part of your storey extension project.
Does my extension need building regulations approval?
Almost all extensions require building regulations approval. Building regulations still apply even without planning permission due to permitted development rights. Extensions are exempt from building regulations only in very limited circumstances. For example, small detached buildings under 15m² with no sleeping accommodation. They must be located more than one metre from boundaries.
Residential house extensions require building regulations approval regardless of size. This applies whether or not you need planning permission. You must submit a full plans application to Building Control before work starts. Or use the building notice procedure. Detailed drawings are provided as work progresses. Most structural engineers and architects recommend full plans applications. These provide clarity and certainty about your storey extension.
Working without building regulations approval is a criminal offence. This can result in enforcement action. You may face expensive remedial works or even demolition. When selling, solicitors require building regulations completion certificates. Their absence can delay or collapse property transactions.
Check the planning portal if you’re unsure. Speak with your local planning authority about whether you need to apply for planning permission alongside building regulations. For a listed building or properties in conservation areas, you’ll almost certainly need for full planning permission. This is in addition to building regulations approval.
Consider getting a lawful development certificate. This confirms your extension benefits from permitted development rights. No need for planning consent. It gives you peace of mind for future property sales.
Need expert guidance on building regulations for your extension? At AC Design Solutions, our chartered architectural technologists and structural engineers provide comprehensive compliance services across London. We cover initial feasibility assessments to detailed structural calculations and Building Control liaison. We ensure your house extension meets all regulatory requirements efficiently. We guide homeowners through every step whether you’re planning a single storey rear extension, a side extension, or a larger extension project. We determine if you need planning permission for a single storey extension. We ensure you comply with building regulations. Contact us for a consultation on your building project.
