Last Night's Viewing: Getting On, BBC4 Grand Designs, Channel 4

I'm at a bit of a loss as to how to explain why Getting On is so good. If you don't like it – and the mere existence of comedies such as Not Going Out and Me and Mrs Jones suggests there must be people who prefer their sitcoms less subtle – then it's hard to know where to start exactly.

Yes, there are funny lines, but their humour tends to be quiet and understated and dependent on character, so that they don't always read as well in cold print. "I'm quite good on the DS," said Nurse Kim optimistically as she prepared to adjust a hi-tech bed in last night's episode, quite untroubled by the fact that she didn't have a clue what any of the buttons were going to do, or the fact that her comatose patient was going to be an unwitting participant in the experiment.

At one point, she nearly had her folded in half before hitting the red button that returned everything to manual. And I suppose that is funny in retrospect – as long as you can take a bit of black indignity – but it still doesn't quite pin down the tone of that scene. There's no studio laughter to provide guide rails for the uncertain viewer and the physical comedy is never flailingly over-done. It never pretends that things are worse than they are to force the laugh. It also doesn't give a damn that you might not know that a DS is a reference to a kind of games console. The character wouldn't say "Nintendo DS" so Jo Brand doesn't either.

It does acknowledge, though, that comedy will often co-exist with melancholy. The scene I'd point a doubter to (and I have a feeling it wouldn't do my cause any good at all) came right at the end. The set-up was this: two hospitals have merged and all the staff are getting Health MOTs. Sister Den returns from hers looking shattered, not, as we discover later, because she's been given an HIV scare by her little fling with Hilary, but because she's found out she's pregnant. And we discover this after a calculated shaggy-dog delay: Kim says goodnight as she ends her shift and walks away down the corridor. Then she comes back again, having noticed that Den is looking troubled and asks whether she's all right. Reassured, she walks away again, even further down the hall, then stops again and – rather wearily – comes back again. It's a little drama of conflicted conscience and friendship that teases out the reveal and adds depth to both characters.

Getting On has its broad laughs – the ghastly breeziness of Dr Moore as she discusses her latest research programme for example: "I just think that vulvas could be a very interesting area for you to get into," she tells a young houseman as she tries to press him into assisting her. But it really works because of the moments where they don't worry too much about getting a laugh at all.

Grand Designs celebrated its 100th anniversary with a spectacular instance of the ingredient that has sustained the series for so long – overreaching folly that succeeds against the odds. Leigh and Graham had taken on a redundant Victorian water tower in Lambeth and were planning to convert it into a home, complete with a brand-new lift tower and "prospect room" with 360-degree views of south London.

"Nothing's going to go wrong," said Leigh. Thirty minutes after the groundwork began, the cost shot up £25,000 when the builders discovered the ground was rubbish. But after that, things proceeded with uncanny smoothness, lubricated by expenditure of £35,000 a week. They pretended there were hitches and Leigh agonised on camera about money, but in truth things went relatively smoothly. Not sure that Leigh can actually afford to live in it now it's finished, but that often happens with castles in the air.

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