If God is in the details, then there are one heck of a lot of buildings going up devoid of the least sign of the divine presence …
As anyone who visits the current Mies Van Der Rohe exhibition at London's Whitechapel Gallery will be reminded, the favourite aphorism of that first great exponent of steel and glass architecture was "God is in the details".

It is good to remember this when so many excellent architects are sidelined from projects once their initial design work has been done, and replaced with jobsworths of one sort or another. A recent casualty is the Richard Rogers Partnership at Terminal 5.

Mies is a brilliant example of his own dictum. If you employ him to work out a minimalist steel and glass office block, you end up with the achingly perfect Seagram building. When you employ AN Other who gets the general idea, but doesn't understand the details, you end up with Croydon.

I don't think it's just a matter of money. Certain clients and builders appear to resent the architect having the final say, or at least to resent being told what is important in their projects. Building jobs have two key stages when details are vulnerable. One is when everyone is trying to get the thing started, and the other when they are trying to get it finished.

Often an innocent sounding decision is made to reconcile budget and tender that has disastrous long-term results. "If we omit the concrete sub sills we can have flush facing windows, which we'd better get in PVCu as they're a bit more exposed." This sort of decision might save £45 a window, at which point the client, contractor and QS breathe a sigh of relief while the architect goes and shoots himself as he sees his carefully modelled elevation destroyed.

In any form of design, particularly in architecture, detail is not an optional extra; it is the whole bloody point

The other stage is when everybody is trying to get the thing finished. "This ironmongery that's been recommended comes from Denmark and is on a 10-week delivery; we can get something more or less the same from our usual supplier." This sort of thing often doesn't save any money and sometimes doesn't even save any time – just a bit of effort. Yet the whole feel and look of the building is spoiled every time you see or use a handle, a hinge, or a hook.

In a cost-saving exercise on the London Eye, the slatted hardwood seats in the pods found themselves in the firing line as bright orange PVC municipal park seats hovered in the office with their "£75 off" labels fluttering. As it happened, a diligent and resourceful architect on the original design team got wind of this and managed to locate a retired French furniture maker operating out of a garage in the Midi who could make them as originally designed for the budget price. In the event, the natural timber proved a reassuring organic element as anxious passengers found themselves swooning with vertigo. Now, supposing that aspect of the finish had only been discovered after the wheel had been spinning for a week? What price the saving then?

In today's "builder in charge of the universe" climate, architects are expendable – except for golf course architects who do what they're told. And BAA probably doesn't need difficult, committed architects to fit out its maintenance hangars or the fire security units. But Terminal 5 is the newest terminal in the busiest international airport in the world. I mean, showcase for British design talent or what? What is the point of opting for Richard Rogers Lite? OK, the detailing in the Lloyd's building is gloriously over the top, but the client that paid for it is not exactly a charity, and was delighted with the result. The same firm's recently completed building in Wood Street shows what can be done with a far less extravagant budget, but where the architect is still fully in control.

It's a bit like asking Jamie Oliver to organise you a big banquet, choose the menu for you, buy the ingredients, and then telling him you've decided to save a few bob by getting your in-house canteen staff to cook it and serve it.