Start with the details the photographs leave out

A bed and breakfast in Marlow can look identical to a bed and breakfast in Henley on a smartphone screen. Wide-angle lenses flatter small rooms, filtered light hides tired carpets, and the garden usually looks larger than it is. The difference between a restful stay and a mildly irritating one rarely comes down to décor. It comes down to the practical things: how much space you actually get, when breakfast is served, and what happens if your plans change. Spend ten minutes checking those three areas before you book and you will almost always end up somewhere better suited to you.

Room size, layout and where the room actually is

Ask for the dimensions in metres rather than relying on adjectives. A room described as "cosy" in a Thames Valley townhouse is often around 10 to 12 square metres, which is perfectly comfortable for one person and a tight fit for two with a fortnight's luggage. A "spacious double" should be nearer 16 to 20 square metres. If you are travelling with a dog, a child, or a mountain of walking gear, this matters more than the thread count.

• Bed size: a standard double in an older riverside property may be a genuine four-foot-six rather than a king. If you are used to a super king at home, ask.
• En suite or private bathroom: these are not the same thing. A private bathroom across the landing is fine in the middle of the night only if you are comfortable with it.
• Location within the building: rooms above the bar, beside the kitchen extractor or facing the A404 will be noisier. Rooms in an annexe or converted coach house can be lovely but may mean a short walk outside to reach breakfast.
• Floor level: many Marlow and Cookham guest houses are period buildings with steep, narrow staircases and no lift. Say so early if stairs are difficult.

Breakfast: timing is the deal-breaker

Breakfast is the one part of the stay that is genuinely hard to work around. Some guest houses serve a single sitting at 8.30am sharp, which is charming until you have a 9am meeting in Maidenhead. Others offer a flexible window from 7am to 10am, or a continental tray left in your room the night before if you are heading out early for a walk along the Thames Path.

Check the following before you commit:

• Service window: what are the earliest and latest times, and do they change at weekends?
• Style: a cooked-to-order full English is a different proposition to a buffet, and a buffet that has been sitting under a lamp since seven is a different proposition again.
• Dietary needs: vegetarian, vegan, gluten-free and dairy-free requests are usually easy if mentioned at the time of booking and awkward if mentioned on arrival. A good host will ask; a great one will already have thought about it.
• Early departures: if you need to leave before breakfast starts, ask whether a packed breakfast or a help-yourself arrangement is possible. Many places will oblige happily if asked in advance.
• Children and portions: if you are travelling as a family, ask whether children's breakfasts are offered and whether there is a charge.

Cancellation terms, deposits and payment

Small guest houses operate on thin margins, so their policies are often stricter than a large hotel's. That is entirely reasonable, but you should understand exactly what you are agreeing to before you hand over card details.

• Free cancellation window: commonly 48 hours to seven days before arrival. If your trip depends on the weather or on a flight, seven days is a long commitment.
• Deposits: many places take the first night or the full amount at booking. Check whether it is refundable and how quickly refunds are processed.
• Amendments: can you move your dates rather than cancel outright? Some hosts will happily shift a booking once; others will not.
• No-shows and late arrivals: if you might arrive after 9pm, confirm that someone will be there to let you in. Not every guest house has a 24-hour desk, and there is nothing worse than a locked door after a long drive.
• Payment method: card, bank transfer, or cash on arrival. Some smaller places pass on card fees.

Location, parking and the small practicalities

In Marlow, Henley and the villages along the river, parking is the most common source of quiet frustration. A guest house advertised as "central" may have no car park at all, leaving you to hunt for spaces in a pay-and-display on the high street. Always ask whether parking is on-site, off-site, reserved for you, and included in the price.

Consider the walk into town too. Fifteen minutes on a summer evening along the river into Marlow is a pleasure; the same walk in the rain with shopping bags is not. If you are relying on trains, check the distance to the station and whether the route is lit at night.

Questions worth asking before you book

A short, friendly email to the owner will tell you a great deal. Ask about room measurements, breakfast times, parking, cancellation terms and whether they can accommodate any specific need. The speed and warmth of the reply is often the best indicator of the hospitality you will receive. A guest house that answers thoughtfully is usually one that looks after its guests thoughtfully, and in the Thames Valley there are plenty of those to choose from.