A Landlords Beginner’s Guide to HMO Conversions in London
Converting a house into a house in multiple occupation (HMO) can earn far more than a single let, but in London it is also one of the most regulated projects a landlord can take on. Planning rules change from borough to borough, licensing is strict, and fire safety standards are higher than for a family home. This guide to HMO conversions in London explains each step in plain English, from checking whether you need planning permission to getting your licence, so you can judge whether a property will work before you buy or build.
We are architects who design HMO conversions across London. If you already have a property in mind, our London HMO conversion service page explains how we can help.
HMO conversions in London at a glance
An HMO is a property let to 3 or more people forming more than one household who share a kitchen, bathroom or toilet.
A small HMO (use class C4) has 3 to 6 occupants. Converting a family house (C3) to C4 is usually permitted development, unless the council has an Article 4 Direction. Many London boroughs do.
A large HMO (sui generis) has 7 or more occupants and always needs full planning permission.
Any HMO with 5 or more occupants needs a mandatory HMO licence. Many London boroughs also license smaller HMOs through additional licensing.
Licensed HMO bedrooms must be at least 6.51m² for one adult and 10.22m² for two. Many boroughs set larger sizes.
A change of use to an HMO needs building regulations approval, with particular focus on fire safety and sound insulation.
What is an HMO?
A house in multiple occupation is a home shared by people who are not from the same household. A household is a single person, a couple or members of the same family. Five friends sharing a house is an HMO. A family of five is not.
C4 or sui generis: which type of HMO are you creating?
The number of occupants decides the planning use class, and that decides how much permission you need.
Small HMO (C4) | Large HMO (sui generis) | |
Number of occupants | 3 to 6 | 7 or more |
Change of use from a family house (C3) | Usually permitted development | Always needs full planning permission |
Where an Article 4 Direction applies | Needs full planning permission | Needs full planning permission |
Mandatory HMO licence | Needed if 5 or 6 occupants | Always needed |
Additional licensing | Depends on the borough | Covered by the mandatory licence |
Article 4 Directions and HMOs in London
An Article 4 Direction removes the permitted development right to change a house into a small HMO. Where one applies, even a 3-person HMO needs full planning permission.
Article 4 Directions for HMOs are common in London. Some boroughs apply them to the whole borough, while others cover only certain wards or areas with lots of existing HMOs. Boroughs including Brent, Newham, Waltham Forest and Redbridge have introduced borough-wide directions, and many others have partial ones.
Because these directions change, always check the current position on your council's planning website, or ask us to check for you, before you exchange contracts. Buying on the assumption that a house can become a C4 HMO is one of the most expensive mistakes new HMO investors make.
If you do not need planning permission, we still recommend applying for a Lawful
Development Certificate. It confirms the HMO use is lawful and helps when you refinance or sell. For more on the planning side, see our planning application drawings service and our guide to planning applications in London.
HMO licensing in London
Licensing is separate from planning. You can have planning permission and still be unable to let the property without a licence. The gov.uk guidance on HMO licences explains the national rules.
Mandatory licensing applies across England to any HMO with 5 or more people forming 2 or more households.
Additional licensing lets councils require licences for smaller HMOs, typically those with 3 or 4 occupants. Most London boroughs run an additional licensing scheme in all or part of their area.
A licence usually lasts up to five years. The council will set conditions, such as the maximum number of occupants and the minimum facilities. Letting an HMO that should be licensed without a licence is a criminal offence. Councils can issue civil penalties of up to £30,000, and tenants can apply for rent repayment orders.
The licence application, step by step
Check which licensing schemes apply in your borough.
Design the layout to meet the council's HMO amenity and space standards.
Prepare floor plans showing room sizes, fire doors, alarms and escape routes.
Gather the gas safety certificate, electrical installation condition report (EICR), energy performance certificate (EPC) and a fire risk assessment.
Submit the application and pay the fee.
Host the council inspection and carry out any required works.
Receive your licence and keep to its conditions.
Designing an HMO that passes planning, licensing and building control
Good design is what turns a house into a compliant, lettable HMO. These are the areas we focus on.
Room sizes and space standards
National rules set minimum bedroom sizes in licensed HMOs: 6.51m² for one person over 10, 10.22m² for two people over 10, and 4.64m² for a child under 10. Any part of a room with a ceiling height under 1.5m does not count towards the floor area, which matters in loft rooms. Most London boroughs set their own HMO standards that ask for more, especially where there is no separate communal living room. See our guide to housing space standards in London for related guidance.
Kitchens and bathrooms
Councils set ratios for shared facilities, typically based on the number of occupants per bathroom, toilet and kitchen. Requirements vary by borough, so we design to the local HMO standards from the first sketch rather than adjusting later.
Fire safety
Fire safety is the biggest design and cost item in most HMO conversions. Expect to need:
an interlinked, mains-powered fire alarm and detection system, usually to a higher grade than in a family home
30-minute fire doors (FD30) with self-closers to bedrooms, kitchen and the escape route
a protected escape route, usually the hallway and stairs, with fire-resisting walls and ceilings
emergency lighting where the escape route needs it
a written fire risk assessment, reviewed regularly.
Sound insulation and ventilation
Separate households living side by side need good acoustic separation. Building regulations require sound insulation between rooms created by a material change of use. Each room also needs adequate natural light and ventilation, and each kitchen and bathroom needs extract ventilation.
Adding bedrooms with a loft conversion or extension
Many London HMO conversions add space, not just reconfigure it. A dormer loft conversion can add one or two en-suite bedrooms on a typical terrace, and a hip to gable loft conversion works well on semis and end-of-terrace houses. A rear extension can give you the larger communal kitchen and living space councils expect as occupant numbers rise.
Two points to keep in mind. First, adding bedrooms can push a C4 HMO to 7 occupants, which makes it sui generis and triggers full planning permission. Second, a loft room adds a storey to the escape route, so the fire strategy must cover the whole house. Our guide to loft conversions in London covers loft types, costs and planning in more detail.
Building regulations for HMO conversions
Changing a house to an HMO is a material change of use, so you need building regulations approval even where planning permission is not needed. Building control will check:
structure, including any new openings, steel beams or loft floors
fire safety, alarms and means of escape
sound insulation between rooms
ventilation, drainage and new bathrooms
insulation and energy performance of any new or altered elements.
Our in-house team prepares the building regulations drawings and structural calculations, so you do not need to appoint a separate engineer. When the work is signed off, you receive a completion certificate, which the council will want to see for your licence.
HMO conversion timeline
Stage | Typical duration |
Feasibility, Article 4 check and measured survey | 1 to 3 weeks |
Design and planning drawings | 3 to 6 weeks |
Planning application or Lawful Development Certificate | 8 to 10 weeks (longer if refused or appealed) |
Building regulations drawings and approval | 4 to 8 weeks, often alongside planning |
Building work | 10 to 20 weeks, depending on scope |
HMO licence application and inspection | Varies by borough, often several weeks or months |
These are typical London timescales, not guarantees. Your programme depends on the property and the council.
Budgeting for an HMO conversion
Costs vary widely, so get a fixed-price quote from a builder with HMO experience once your drawings are complete. Your budget should cover professional fees, planning, building control and licence fees, fire safety works (which new landlords often underestimate), furniture for each room and the communal areas, and a contingency of 10 to 15% for surprises in older houses.
Letting your HMO under the Renters' Rights Act
The Renters' Rights Act 2025 changes how private tenancies work in England. Assured shorthold tenancies have been replaced by periodic assured tenancies, and section 21 "no-fault" evictions have ended. If you are planning an HMO, build these changes into your letting strategy and speak to a letting agent or solicitor about current tenancy agreements. You still need to protect deposits, carry out Right to Rent checks and keep gas, electrical and EPC certificates up to date.
Frequently asked questions
Do I need planning permission for an HMO conversion in London? For a small HMO (3 to 6 people), not always. It is usually permitted development unless the council has an Article 4 Direction, and many London boroughs do. A large HMO for 7 or more people always needs full planning permission.
What is the minimum bedroom size in a London HMO? The national minimum in a licensed HMO is 6.51m² for one adult and 10.22m² for two. Most London boroughs set higher standards, so check your council's HMO amenity standards.
Do I need an HMO licence for 3 or 4 tenants? Mandatory licensing starts at 5 occupants. But most London boroughs run additional licensing schemes that cover smaller HMOs, so you may still need a licence for 3 or 4 tenants.
Can I add a loft conversion to an HMO? Yes, and it is a common way to add bedrooms. Check whether the extra rooms take the HMO to 7 or more occupants, and make sure the fire strategy covers the new storey.
How long does an HMO conversion take in London? Design and approvals often take three to six months, and the building work typically takes 10 to 20 weeks. Licensing can add more time, depending on the borough.
Do I need an architect for an HMO conversion? It is not a legal requirement, but HMOs involve planning, licensing, fire safety and building regulations all at once. An architect who knows London borough standards can reduce the risk of refusals, redesigns and delays.
Plan your HMO conversion with Excela Architecture London
A successful HMO conversion in London starts with the right checks before you buy, and a design that meets planning, licensing and building regulations from day one.
Excela Architecture London designs HMO conversions across London. We handle feasibility and Article 4 checks, measured surveys, planning and Lawful Development Certificate applications, building regulations drawings and structural calculations in-house, and we prepare the floor plans you need for your licence. Find out more about our HMO conversion service, or book a free consultation to talk about your property. You can also call us on 020 8064 4098.
