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How to Split a House into Two Flats – Planning and Legal Steps Explained





Splitting a single residential home into two self-contained flats can be a smart investment move, especially in high-demand areas like West London. Whether you’re a landlord looking to maximise rental income or a homeowner hoping to make use of unused space, it’s important to understand the legal, planning, and design implications involved.


At SY Design Studio, we’ve helped many homeowners and developers navigate this process successfully. Here’s a complete guide to what’s involved.





✅ 1. Do You Need Planning Permission?



Yes — in almost all cases, splitting a house into two flats requires full planning permission. This is because it’s considered a material change of use from a single dwelling (C3 use class) to multiple dwellings (sui generis or C3 multiple units, depending on council classification).



Things to consider:



  • Check your council’s Local Plan for any restrictions on conversions (e.g. Hounslow, Ealing, and Brent all have different policies).

  • Some councils may resist flat conversions in areas where housing mix or family homes are prioritised.

  • Article 4 Directions may remove permitted development rights — so planning is always required.






📝 2. What Documents Are Required for the Planning Application?



To submit a successful planning application, you’ll need:


  • Existing and Proposed Floor Plans

  • Proposed Elevations (if external alterations are planned)

  • Location and Site Plan

  • Planning Statement (explaining how the proposal complies with policy)

  • Design & Access Statement (for more detailed applications)



Optional but recommended:


  • Parking Layout or Transport Statement

  • Refuse and Cycle Storage Details

  • Daylight/Sunlight Assessment (if neighbouring light might be affected)



SY Design Studio handles all of this for you — from measured surveys to planning submission.





🏠 3. What Design Standards Apply?



Councils usually require:


  • Minimum floor areas as per the Nationally Described Space Standards (NDSS)

  • Adequate internal layout with bedrooms, bathrooms, and kitchens for each unit

  • Separate entrances (or secure and clearly defined shared access)

  • Bin and cycle storage

  • Sufficient natural light and ventilation for all habitable rooms






⚖️ 4. Building Regulations Approval



Even if planning is approved, you still need Building Regulations approval to ensure the conversion meets safety, fire, insulation, drainage, and soundproofing standards.


Key areas include:


  • Fire protection and escape routes

  • Sound insulation between flats

  • Drainage alterations

  • Electrical rewiring and ventilation



We provide full Building Regulation drawings and structural calculations to ensure your project passes with no delays.





💷 5. What Are the Costs Involved?



Here’s a breakdown of typical costs:


  • Planning Drawings & Application: From £950–£1,500+

  • Council Planning Fees: £258 (householder) or more depending on scope

  • Building Regulations Drawings: From £950+

  • Structural Engineer Fees: From £400–£700

  • Construction Costs: £25,000–£60,000+ depending on complexity and size






📬 6. Do You Need to Notify Freeholders or Mortgage Lenders?



If the property is leasehold, you’ll need consent from the freeholder.


If you have a mortgage, splitting a home into flats might breach your mortgage terms — so always inform your lender before starting works.





🧾 7. Other Legal Steps



  • New utility meters may be needed for gas, electricity, and water

  • You’ll need to apply for separate council tax bands

  • Once complete, register each unit with the Land Registry

  • If renting out, ensure HMO licensing doesn’t apply (usually 3+ unrelated tenants per flat)






🛠️ How SY Design Studio Can Help



At SY Design Studio, we provide an end-to-end service:


  • Site survey & concept advice

  • Planning drawings & submission

  • Building Regulations + structural design

  • Guidance on legal and compliance issues



We’ve worked on successful flat conversions across West London — including in Hounslow, Brentford, Ealing, and Isleworth.





📞 Book a Free Consultation Today



Thinking of converting your house into two flats?

Let’s chat about your project and how we can help you maximise its value.


👉 Call us on 07859 431 227

 
 
 
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