Converting a house to an HMO in Cardiff

HMO conversion is one of the most reliable routes to above-average rental yields in Cardiff, but it involves a sequence of decisions that must be taken in the right order. Planning, finance, room layout, fire compliance, licensing and the correct occupation contracts under the Renting Homes (Wales) Act 2016 all connect: getting one wrong adds delay and cost to every stage that follows. Landlords who approach conversion systematically consistently achieve better outcomes than those who start the refurbishment first and deal with compliance afterwards.

This guide covers the full HMO conversion process for Cardiff properties, from the initial planning due diligence through to the moment you hand a key to your first contract-holder. It covers the specific considerations that apply in Cardiff, including the Welsh planning position that requires permission for every small HMO conversion and the licence conditions set by Cardiff Council that go beyond the national minimum. An interactive conversion phase timeline at the end lets you step through each stage in detail.

Step 1: Check the planning position before you buy

Planning use class determines what can be done with a property without permission. A standard house sits in class C3. A small HMO of three to six unrelated occupants is class C4, a class introduced in Wales on 25 February 2016. Seven or more occupants falls outside both classes into what is called sui generis.

This is the point where Welsh and English practice diverge, and where a great deal of online guidance is simply wrong. In England, moving from C3 to C4 is a permitted development right, and English councils use Article 4 Directions to remove that right in specific streets or wards. Wales never granted the right in the first place. The Town and Country Planning (General Permitted Development) (Amendment) (Wales) Order 2016 allows movement from C4 back to C3 only, never the other way.

The practical consequence for Cardiff is simple and it applies citywide: planning permission is required to convert a C3 house into a C4 small HMO anywhere in Cardiff, whether the property is in Cathays, Heath, Grangetown or Llanishen. There is no "outside the Article 4 area" exemption in Wales, because there is no permitted development right to be removed. Seven or more occupants likewise always requires express permission. What does vary by area is how likely permission is to be granted: Cardiff Council manages HMO concentration through its supplementary planning guidance, applying a threshold of roughly 20% of dwellings in Cathays and Plasnewydd and 10% elsewhere, measured within a short radius of the property. OurCardiff HMO planning guideexplains the density rule and how to carry out the due diligence check before you make an offer.

Step 2: Secure the right finance

A standard buy-to-let mortgage is not written for HMO use and some lenders will void the policy if the property is used as one. Most Cardiff investors fund the purchase and conversion using a bridging loan, then exit into a longer-term specialist HMO mortgage once the property is licensed and producing income. HMO lenders assess properties differently: some base the loan on a per-room income valuation, others on a single-property figure. Understanding your lender's approach before you agree a purchase price is essential, as it directly determines your maximum affordable bid. For the yield benchmarks lenders typically apply when stress-testing HMO income, see our guide toHMO yields in Cardiff.

Step 3: Design to Welsh room size and amenity standards

Cardiff Council's HMO amenity and space standards set minimum floor areas for sleeping rooms alongside kitchen and bathroom provision. Crucially, the bedroom minimum depends on whether the house provides a separate living room. Where it does, a single room must be at least 6.5 square metres and a double at least 11 square metres. Where it does not, the bedroom has to absorb that living space and the minimums rise sharply — 10 square metres for a single and 15 square metres for a double. These are the floors accepted at inspection, and only usable floor area counts. Commercially, designing to 8 to 12 square metres for singles produces rooms that let faster and at a premium in Cardiff's student and professional market. Kitchen provision scales with occupant number: a shared house needs at least 7 square metres of kitchen for up to six people, plus 2.5 square metres for each additional occupant, with one sink per five occupants and two linear metres of worktop per five. A separate living room must be at least 11.5 square metres for up to six people. Cardiff sets no minimum size for bathrooms, requiring only enough space to change and dry comfortably. Be aware that a conversion may also face separate planning and building control space standards on top of these.

Step 4: The refurbishment phase

HMO refurbishment goes well beyond a cosmetic update. Structural changes to create additional bedrooms will require building regulations approval in most cases. Fire compartmentation between floors, fire stopping around pipe penetrations and the correct construction detail at party walls must be built into the fabric of the building from the start, not retrofitted once rooms are boarded. The electrical installation must be specified for HMO use, typically with a dedicated consumer unit per floor and circuits sized for the occupancy level, certified under Part P. A full breakdown of what Cardiff HMO refurbishments typically cost, and where the budget goes, is in ourHMO refurbishment costs guide.

Step 5: Fire and electrical safety sign-off

Fire safety compliance must be addressed during the build, not after. FD30 self-closing fire doors with intumescent strips and smoke seals on the frame must be fitted to all bedroom and kitchen doors opening onto the escape route. Mains-powered interlinked smoke alarms must cover every storey and corridor. Emergency lighting in corridors and stairwells is a standard Cardiff Council licence condition for properties above two storeys. Appointing a fire risk assessor before boarding out allows any deficiencies to be corrected at minimal cost. Our HMO fire safety in Wales guidecovers every element in detail. The EICR from the Part P electrician must be current and available at the point of licence application.

Step 6: Apply for your HMO licence before letting

Once the conversion is substantially complete, the licence application can be submitted to Cardiff Council. A licence must be in place before any contract-holder moves in: operating without one is a criminal offence under section 72 of the Housing Act 2004, punishable on summary conviction by an unlimited fine, and contract-holders can reclaim up to 12 months' rent through a rent repayment order. The application requires a floor plan showing room sizes, evidence of planning compliance or confirmation it is not required, current fire and electrical safety certificates, and a gas safety certificate if the property has a gas supply. Cardiff Council may inspect the property before issuing the licence. OurHMO licensing guidecovers both mandatory and additional licensing in full.

Step 7: Set up occupation contracts under Welsh law

Under the Renting Homes (Wales) Act 2016, rooms in a Welsh HMO are let under occupation contracts, not the Assured Shorthold Tenancy framework that still applies in England. Each room requires its own occupation contract. Contract-holders must receive a written statement within 14 days of moving in. The written statement must use the model published by the Welsh Ministers or a document that is compliant with it. Where bills are included in the room rent, the amount covered must be stated. For landlords new to the Welsh framework, theRent Smart Waleslandlord training covers occupation contract obligations in practical terms, and theWelsh Government's renting homes guidancesets out the statutory requirements.