Converting A Garage Or Loft Into A Holiday Let In Cornwall

Plenty of people in Cornwall have worked out that a spare room can pay for itself. What has changed is everything around it, and most of it changed in 2025. Cornwall Council now charges a 100% council tax premium on second homes, the furnished holiday lettings tax regime was abolished in April 2025, and the threshold for being treated as a business rather than a second home sits at 140 nights available and 70 nights actually let. None of that stops a garage or loft conversion earning money. It does mean the room has to be built and run differently from one you were only ever going to put a relative in.

The room stops being yours, and that changes the spec

A conversion built for family use gets judged by one person, and that person forgives things. A holiday let gets judged by strangers, in writing, publicly, within a week of staying. That single difference drives most of the decisions.

Practically, it means the finish has to survive turnover rather than just look right on handover. Flooring gets dragged suitcases rather than slippers. Sockets need to be where a guest will look for them, not where the wiring was convenient. Heating has to come up to temperature quickly, because guests arrive at four in the afternoon in February and judge the place in the first ten minutes. And the room needs to be warm and dry without anybody thinking about it, which for a garage conversion in Cornwall means the floor build up and the wall insulation get specified properly at the start rather than argued about later. Guests do not give you the benefit of the doubt. They give you four stars.

Ancillary use, or a separate dwelling?

This is the line that decides whether you need planning permission, and it has nothing to do with the building work.

If the converted space is used as part of your house, and guests come and go through your front door, you have one dwelling being let. Letting your own home while you are away is the same thing. Nobody needs an application for that.

The picture changes when the space becomes self-contained and is occupied independently of the main house. Its own kitchen, its own entrance, its own outside space, and someone living in it who has nothing to do with you, and the argument that it is still ancillary to the house gets thin. That is the point at which Cornwall Council can treat it as a material change of use and expect an application. Detached garages sit closest to this line, because they are already a separate building with a separate door. If what you actually want is a self-contained unit, look at it as an annexe or granny flat from the start and get the use agreed, rather than building a room and finding out afterwards what it counts as.

England still has no dedicated planning use class for short term lets in force. It has been consulted on, and a national registration scheme has been promised, but neither has commenced. Do not build a scheme around a rule that has not arrived.

Building control does more of the work than planning

Almost every conversion enquiry starts with planning permission, and almost every conversion is decided by building regulations instead. Turning a garage or a loft into a habitable room is a material change of use under the regulations whatever you intend to do with it, and that consent is not optional.

For a letting room the parts that bite hardest are fire, insulation and ventilation. Escape and detection come under Part B. The thermal upgrade of walls, floor and roof comes under Part L, and a garage built as a garage will not be close. Background ventilation and extract come under Part F, which matters more than owners expect in a room that will hold two or four people overnight with the windows shut. Add a shower and Part G brings hot water and scald protection into it.

The paperwork at the end is worth as much as the room. A completion certificate is what a letting agent, an insurer and eventually a buyer’s solicitor will ask for, and the buyer’s solicitor is the one who asks at the point where you have already accepted an offer.

Fire is where a holiday let parts company with a family home

Your own house is not covered by fire safety law in the way a business is. A holiday let is. The moment you take paying guests, the Home Office guidance on small paying guest accommodation applies, published under the Regulatory Reform (Fire Safety) Order 2005. You become the responsible person. You have to carry out a fire risk assessment, act on it, and keep it.

That guidance is written for exactly the size of premises most Cornish conversions produce: single properties of ground floor, or ground and first floor, sleeping up to ten people, with no more than four bedrooms on the first floor. Cottages, individual flats and converted outbuildings all sit inside it.

What it tends to mean in practice is mains powered, interlinked detection through the whole property rather than a single battery alarm in the converted room, a clear escape route that does not run through a kitchen, doors that hold long enough for someone unfamiliar with the layout to get out, and instructions a guest will actually read. None of it is expensive if it is designed in. All of it is expensive if it is retrofitted after the plastering.

Council tax, business rates and Cornwall’s premium

This is the part that changes the numbers most, and it is the part people research last.

Cornwall Council has charged a 100% premium on second homes since 1 April 2025, which means a second home pays twice the standard bill. If your conversion is part of your main residence this does not touch you at all. If it forms a separate property that sits empty most of the year, it very much does.

The route off council tax is business rates, and the thresholds are national. A self catering property in England is assessed for business rates when it is let commercially in periods of 28 nights or less, was available to let for at least 140 nights in the last 12 months, was actually let for at least 70, and will be available for 140 nights again in the coming year.

Seventy nights is the one that catches people. Availability is easy to prove because it is your own calendar. Actual occupancy is not, and a room that only fills in July and August will not get there. Work out honestly whether you will hit it before you build a business case on the assumption that you will.

The tax rules changed in April 2025

The furnished holiday lettings regime, which for decades gave holiday lets their own treatment for capital allowances, mortgage interest and capital gains, was abolished with effect from April 2025. Income and gains from a holiday let now sit inside your ordinary property business and are treated like any other property income.

We are builders, not accountants, and this is a paragraph to take to someone qualified rather than act on. The reason it belongs in a conversion article is simply that a lot of the older advice circulating about how well a holiday let pays was written under rules that no longer exist. If you last looked at this in 2023, look again before you commit to the spend.

Garage or loft, and what a lettable room costs to build

For letting specifically, the garage usually wins, and the reason is the front door. A converted garage can be given its own entrance, its own services and, if it is detached, complete separation from the house. A loft room cannot. Guests reach it through your landing, past your bedrooms, and the escape route is shared. Lofts make excellent holiday accommodation when the entire house is let, and awkward accommodation when it is not.

On cost, a good integral garage conversion in Cornwall runs from around £11,000 to £18,000 depending on the floor build up, the glazing and whether a shower room goes in. A detached garage brought up to habitable standard sits higher, because services have to be run across the garden and the structure is often single skin. A loft conversion is a different order of money again, generally £30,000 upwards once the stair, the structure and the fire work are counted.

Against that, the letting income is seasonal and the running costs are not. It is worth being straight with yourself about the second use of the room, because a conversion that only makes sense at full occupancy is a fragile investment, whereas one that works as a bedroom or an office in the off season is not. That question of what the room is worth when you are not letting it is the same one we cover in more detail when looking at whether a garage conversion adds value in Cornwall.

If you are weighing up a conversion with letting in mind, the useful conversation happens before the design is fixed, not after. The fire strategy, the entrance, the services and the question of whether you are creating a separate dwelling all cost very little to get right on paper and a great deal to correct in brick. Get a free quote and a straight answer on your property.