Converting a house into two self-contained flats can be an attractive option for homeowners, landlords and property developers looking to create additional accommodation or maximise the potential of an existing property.
However, obtaining planning permission for a house-to-flat conversion in the London Borough of Lewisham is not always straightforward.
Lewisham’s planning policies seek to balance the creation of additional homes with the protection of existing family housing, residential amenity and neighbourhood character.
If you own a property in Lewisham, Brockley, Catford, Forest Hill, Sydenham, Deptford or New Cross, understanding the relevant planning restrictions before commissioning drawings can help avoid costly redesigns and unsuccessful applications.
Do You Need Planning Permission to Convert a House into Two Flats?
Yes. Converting a single dwellinghouse into two or more self-contained flats normally requires full planning permission.
This is because the proposal creates additional independent residential units, rather than simply altering the internal layout of an existing house.
Planning permission is generally required even where the conversion involves no external alterations.
A householder planning application is not appropriate for this type of development. Instead, a full planning application will normally be required.
Lewisham’s Planning Rules for House-to-Flat Conversions
Lewisham adopted its new Local Plan on 16 July 2025, replacing its previous development management policies.
The relevant policy framework includes Policy HO2 – Optimising the Use of Small Housing Sites, alongside other policies concerning housing quality, design and residential amenity.
A central consideration is the protection of family-sized accommodation.
1. The 130m² Floorspace Requirement
Lewisham’s adopted Policy HO2 includes a minimum original gross internal floorspace requirement of 130m² for proposals involving the subdivision of a single-family dwelling or a self-contained dwelling with three or more bedrooms into smaller self-contained homes.
This is an important initial consideration for homeowners contemplating a conversion.
For example:
- A house with an original gross internal floorspace of 110m² would not meet the relevant threshold.
- A house originally measuring 150m² may satisfy the floorspace requirement but must still comply with other planning policies.
- Extending a smaller house does not automatically overcome the requirement relating to its original floorspace.
The measurement of the original dwelling is particularly important where a property has previously been extended.
Meeting the 130m² threshold does not automatically mean that planning permission will be granted.
2. Protecting Family-Sized Housing
Lewisham also seeks to protect the supply of larger homes suitable for families.
Policy HO2 generally requires the retention or re-provision of a self-contained family-sized unit containing at least three bedrooms when an existing family dwelling is subdivided.
This can significantly influence the number and size of flats that can be created.
For example, converting a four-bedroom house into two smaller two-bedroom flats may conflict with the policy.
An alternative arrangement retaining a three-bedroom unit alongside a smaller flat may be more appropriate, provided the proposed accommodation meets the relevant space, design and amenity requirements.
3. Are Exceptions Possible?
Lewisham recognises that some buildings or sites may not be suitable for continued family occupation.
The council may consider exceptions to the requirement to retain family-sized accommodation where environmental or other site-specific constraints justify an alternative arrangement.
Relevant considerations may include:
- Insufficient outdoor amenity space suitable for family occupation.
- The size or configuration of the building preventing a practical family-sized dwelling.
- Environmental conditions that would adversely affect occupants, particularly children.
Any exception must be justified with appropriate evidence.
The potential financial return from creating smaller flats would not, by itself, provide a sufficient planning justification.
Can You Extend a House Before Converting It into Flats?
Homeowners sometimes consider adding a rear extension or loft conversion to increase the available accommodation before subdividing a property.
Although extensions may help create flats that satisfy minimum residential space standards, they do not automatically overcome restrictions relating to the original dwelling.
Where the planning policy refers to original gross internal floorspace, additional accommodation created through later extensions cannot simply be assumed to count towards the 130m² threshold.
The property’s planning history and original configuration should therefore be reviewed before detailed conversion drawings are prepared.
What Are the Minimum Sizes for Two Flats?
New self-contained flats are generally expected to meet the residential space standards incorporated into the London Plan.
The minimum gross internal areas depend on the number of bedrooms, intended occupants and number of storeys.
| Accommodation | Minimum area |
|---|---|
| 1-bedroom, 1-person flat | 39m² |
| 1-bedroom, 2-person flat | 50m² |
| 2-bedroom, 3-person flat | 61m² |
| 2-bedroom, 4-person flat | 70m² |
| 3-bedroom, 5-person flat | 86m² |
These figures are for single-storey accommodation. A one-bedroom, one-person flat may have a reduced minimum of 37m² where a shower room is provided instead of a bathroom.
Larger minimum areas apply to accommodation arranged over multiple storeys.
Other requirements include appropriate bedroom dimensions, storage, natural daylight, ventilation and ceiling heights.
Simply dividing the existing floor area into two equal parts will not necessarily produce acceptable accommodation.
Do Both Flats Need Separate Entrances?
Each flat must have suitable independent access, but this does not necessarily mean that both require separate external front doors.
A common arrangement involves a shared entrance hall with individual entrance doors serving each flat.
Alternatively, the ground-floor flat may have its own entrance, with a separate staircase serving the upper-floor accommodation.
The proposed arrangement must satisfy relevant planning, accessibility and fire safety requirements.
In Victorian terraced houses, the existing entrance hall and staircase configuration can significantly influence the feasibility of a conversion.
Do Both Flats Need Private Outdoor Space?
Outdoor amenity space is an important consideration when assessing residential conversions.
A ground-floor flat may have direct access to the existing rear garden.
Providing appropriate outdoor space for an upper-floor flat can be more challenging.
Possible arrangements include subdividing the rear garden into separate areas or providing suitable shared amenity space.
Balconies and roof terraces may also be considered, although these can introduce additional planning concerns relating to overlooking, privacy and noise.
Where a family-sized unit is retained, Lewisham’s conversion policy expects access to suitable garden space where available, with access for other units encouraged wherever possible.
What About Parking, Cycle Storage and Refuse?
Creating an additional dwelling can increase the demand for parking, cycle storage and refuse facilities.
Lewisham’s approach will depend on the property’s accessibility to public transport, existing parking arrangements and applicable London Plan requirements.
Some developments may be expected to operate without additional car parking, particularly in areas with good public transport accessibility.
Secure cycle parking and suitable refuse storage will normally need to be considered.
These facilities should be integrated into the proposed layout rather than treated as an afterthought.
Building Regulations for House-to-Flat Conversions
Planning permission is only one part of the approval process.
Converting a house into separate flats also involves substantial Building Regulations considerations.
Fire Safety
Fire safety is often one of the most significant technical challenges.
The conversion may require:
- Appropriate fire separation between the flats.
- Fire-resisting floors, walls and doors.
- Suitable means of escape.
- Appropriate fire detection and alarm arrangements.
- Protection of shared entrance halls and staircases.
- Consideration of external fire spread and fire service access.
The appropriate fire strategy will depend on the height, layout and existing construction of the building.
Specialist fire safety advice may be necessary, particularly where the existing arrangement cannot readily accommodate a conventional means of escape.
Sound Insulation
Separating a house into independent dwellings introduces requirements for sound insulation between the flats.
Existing timber floors and masonry walls may require upgrading to reduce airborne and impact sound transmission.
Pre-completion sound testing may be necessary unless an accepted alternative compliance route applies.
Thermal Insulation and Ventilation
Alterations to the building may trigger requirements to improve the thermal performance of relevant construction elements.
Each flat must also have suitable ventilation, heating and hot water arrangements.
The extent of the necessary work will depend on the existing construction and the proposed alterations.
Drainage and Services
Creating an additional dwelling may require new bathrooms, kitchens, drainage connections, electrical installations and independent utility supplies.
The existing drainage system should be assessed to establish whether it can accommodate the additional facilities.
What Drawings Are Required for Planning Permission?
A typical full planning application for a house-to-flat conversion will require:
- Existing floor plans and elevations.
- Proposed floor plans showing the individual flats.
- Proposed elevations where external alterations are involved.
- Site location and block plans.
- Relevant sections where necessary.
- Supporting information addressing residential standards, access, refuse storage and other applicable planning requirements.
Additional documents may be necessary depending on the location and nature of the development.
Accurate measured survey drawings are particularly important when determining whether the proposed flats can meet minimum space standards.
Why Might Lewisham Refuse a House-to-Flat Conversion?
Common planning concerns include:
- Unacceptable loss of family-sized housing.
- Failure to satisfy the relevant original floorspace threshold.
- Flats that do not meet minimum residential space standards.
- Poor natural daylight, outlook or ventilation.
- Inadequate private or communal amenity space.
- Unacceptable overlooking, noise or disturbance.
- Unsatisfactory access, refuse storage or cycle parking.
- Harm to the character of the property or surrounding area.
Identifying these issues before preparing detailed drawings can significantly reduce the risk of an unsuccessful application.
Planning a House-to-Flat Conversion in Lewisham?
At Draw & Plan, we prepare measured surveys, planning drawings and Building Regulations drawings for residential developments throughout South London.
Before providing a quotation, we undertake a desktop review of the property’s planning constraints to identify relevant policies and the likely application requirements.
Where specialist fire engineering, structural calculations or other technical assessments are necessary, these can be considered separately as part of the project requirements.
If you are considering converting a house into flats in Lewisham, send us the property address and a brief description of your proposal.
We can review the relevant planning constraints and provide a quotation for the required drawings.
Further Reading
This article provides general guidance based on the planning framework applicable in October 2026. The acceptability of an individual proposal will depend on adopted policies, planning history, site circumstances and supporting evidence.
Disclaimer: This article is for general informational purposes only and does not constitute legal or professional advice. Always consult relevant professionals and local authorities before undertaking any development or change of use.
