Installing a bathroom in your annexe is a serious undertaking and can be deceptively tricky. It requires quite an array of fittings and technical skill, often in a very compact space. For this reason, carefully planning from the start will prevent costly mistakes and changes down the line.

Assess your bathroom needs
Begin by working out your key priorities. A family bathroom or spa-style sanctuary will require a long list of specifications and fittings that could include a bath, separate shower, double basins, and heaps of storage, while an en-suite or guest bathroom may just require a shower and loo.

Next, ask yourself what sort of space you hope to achieve. Are you seeking a haven to escape to, or are you more of an invigorate-and-energise kind of person? This will affect the fixtures, fittings and colour scheme you choose. Forget about how you currently use your bathroom, and instead imagine how you would like to use it. Draw on your experiences when you’ve visited great spas and visualize how you would like your new scheme to function.

Plan a bathroom layout
If your layout isn’t quite right, think about the ways you might be able to make small but effective changes. Rehanging doors or fitting sliding ones is an excellent way to gain space, for instance.

If you do plan to change your layout, measure your bathroom carefully. Note the position of doors windows and any pipes. It might help to draw an outline on graph paper, or use an online design tool.

It’s also a good idea to visit a local bathroom showroom for advice, as its designers will have plenty of experience in maximising space while at the same time minimising costly mistakes.

Finally, bear in mind how your needs might change in the years to come. Just because you don’t need a bath in your life right now… it might not always be the case. Your family might grow, or indeed you might be selling, in which case a bath will have broader appeal.

Seek out inspiration
Choose your style early on as this will help narrow down the choice of fittings to consider. Your main options are period style, classic or a contemporary look. Hotels, particularly small boutique ones, are a fabulous source of design inspiration, so scour their websites for looks you love. Quite often they’ll feature a lot packed into a compact space, so you’ll probably pick out a few layout tips, too.

Flick through interiors magazines to gather tear sheets and try to bring together a visual checklist of what appeals to you, whether that’s an Eastern-Zen look, or something more Scandi-inspired. This will really help formulate your ideas for the look. It’s easier to show someone a mood board to express your vision.

Talk to friends who have completed similar projects about what they love about their new bathroom, and to discover the pitfalls and problems they may have encountered along the way.

Be realistic about your space
There’s no point pining for his-and-her sinks, a statement walk-in shower or a double-ended roll top if they won’t fit. The truth of the matter is it isn’t going to be possible in the most UK bathrooms the average footprint is about the same as a king-size bed.

Don’t cram in fittings as this will only make the room feel smaller and research products that will help you make the most of your square meterage, such as wet-room style showers or Japanese-style square tubs.

Remember, ventilation is important
People overlook the ventilation in their bathroom, only to regret it later. It’s so important in a bathroom with furniture, as condensation can damage wood, an open window alone is not enough.

You certainly don’t want the bathroom to become damp, especially an en suite placed so close to the bedroom, which is why extraction needs to be considered. Make the room well ventilated, as you certainly don’t want steam flowing in to the bedroom. Nor do you want your extraction to be too loud.

Factor in storage
Storage is the key to a successful bathroom, a minimal glass shelf won’t be enough! Toiletries never display well, nor do shampoo bottles standing on the floor of the shower. A substantial vanity with space for extra towels is a smart buy.

Mirrored cabinets will allow for you to store products, as well as charge toothbrushes and electric razors. Niche storage in the walls will also ensure that you can store bottles discreetly when in the shower.

Consider your bathroom lighting
As bathrooms are often placed next to the bedroom, lighting should be considered carefully. You don’t want bright lights flashing on in the middle of the night as it’s not comfortable for the person taking a trip to the bathroom, or for the person asleep next door bedroom.

I would suggest that you consider having different levels of lighting. Then, if you’re taking a late-night trip to the bathroom, you can turn on the low level lighting without waking up others. Some of the lighting could be operated by a motion sensor, meaning there is no fumbling around for a light switch.

Tiles are the easiest way to follow trends
Surfaces make or break a bathroom: there’s little that feels more dated than cruddy, worn-out vinyl. One current trend is for graphic ceramics – say, a patchwork of decorative geometrics or a striking hexagonal design. You’ll achieve maximum impact with a floor-to-wall seamless feature, but you can also opt for a feature wall or floor. Think about your room proportions before choosing your tiles. Very decorative designs can be too busy for a small space, whereas they could be an ideal way to add interest and warmth to a large bathroom.

Finalise a budget
Deciding on a budget in advance means you can prioritise where to spend and where to save. And the good news is that it doesn’t have to cost thousands to create a spa feel. Invest in just one beautiful piece, such as a freestanding bath which you can pick up for under £500 and show it off against a more budget-conscious backdrop. Bear in mind you also need to allocate some money to practical elements. You might need a larger water tank if you are planning a rainfall shower, and a water softener can be a smart idea to prevent the build-up of limescale.

Decide what level of service you need
This will largely be dictated by your budget. Some bathroom companies offer a turn-key solution, with everything from planning and installation to decorating included in the price. While this isn’t the cheapest option, it can repay in terns of the speed and ease of your project. Alternatively you can design the space yourself, buy your own fittings, and hire a plumber or builder to work to your plan. While you have to ensure you get the layout right, and it will take more time to source products, it should work out cheaper in the long run.

Of course, nothing beats seeing products in the ‘flesh’ so put time in to visit plenty of showrooms. Don’t be afraid to get in the bath and test it for comfort right there in the showroom, and feel for furniture and taps for build quality.

Organise your tradespeople
Get recommendations from friends and family about fitters. Always ask for references and make sure the person you choose is some one you trust and – importantly – like, as they will be in your home for a week or so. Remember, the best price might not always result in the best service.

Buying your suite and fittings through your builder might allow you to take advantage of trade discounts. Some bigger bathroom suppliers offer significant savings to the trade, as well as fast delivery and product support. Don’t forget, though, that your fitter will probably take a small cut for the time and effort of ordering for you.

Regardless of whether you are buying from a high-street store or a high-end brand, factor in delivery times. Big name manufacturers might take three or four weeks to deliver, while luxury brands can take around eight weeks.

When placing your orders, don’t forget the peripherals, such as waste traps for the bath and sinks and try to schedule delivery to coincide with when the builder is on site otherwise you could have items cluttering up the house. Or, worse still a builder with nothing to fit. Give yourself plenty of wriggle room to avoid having contractors twiddling their thumbs on site.

Survive the work
How long you’ll have to cope without a shower or bath depends on the scale of the work – a spruce job might only take a matter of days, but if you are planning to move fittings or relocate plumbing, account for significantly longer.

You’ll also want to take steps to minimise the inevitable mess. Cover stairs and hallway floors with old dust sheets to protect them while products are being taken through the house. Ask for any cutting of tiles and plasterboard to be done outside to minimise dust, and seal off doors to any bedrooms using masking tape to keep them pristine.