As a commercial architect, Philip Leeson has long been a proponent against making rash decisions. That, at least, is his stance on razing private homes with endless potential for innovative designs.

But when it comes to his firm's other long-term function – as manager of the heritage advisory service on behalf of the ACT government –the tables are often turned. It's a role the firm has held for the last 15 years, since the service was introduced into the ACT. Put simply, Leeson and his team advise those who own property or want to live in one of the dozen or so heritage precincts in Canberra, such as Forrest, Kingston, Manuka and Ainslie.

"They're usually precincts that represent certain periods of development within Canberra, and they illustrate the social classes that were built into Canberra," he says.

"So, for instance, in Forrest there were big blocks of land with generous houses and lots of space, and they were houses for senior public servants, and then you go down to Ainslie, and there's little squishy blocks and wooden houses."

His firm, he says, is best placed to provide this advice because of the diverse nature of its practice – general architecture, heritage work, interior design, social housing and master planning. But that doesn't make the job easy, not with the rigid rules that govern Canberra's architectural heritage.

On the face of it, his firm has only to provide succinct advice – dispensed in one-hour consultations – to ensure that owners or prospective owners of heritage sites proceed with caution.

"It's really just to make sure that they get off on the right track, rather than getting some building designer who knows nothing about the constraints or misunderstands the rules," he says.

But it's the rules that can sometimes present problems.

"They're set in stone, really, they're part of legislation and they're really quite clear. Or when I say 'quite clear', there's no room for discretionary interpretation," he says.

"In fact we and some of the heritage people in the heritage unit get a bit frustrated with that. We sometimes see projects with great merit, but because they don't follow the strict rules, the guidelines, they can't be approved."

He gives the recent example of a large, distinctive, heritage-listed house in Manuka that had been examined by a Melbourne architect with a view to transforming it into a hotel. The idea was to keep the original features of the house, and build an adjacent structure in a modern style that echoed the original.

But, says Leeson, the combination of a skilled architect and a viable scheme proposal was not nearly enough to overcome the rules. The council, he says, "simply wasn't interested".

It is partly down to how the legislation has been drafted, but Leeson points out that the ACT – progressive little Canberra, of all places – is particularly conservative when it comes to heritage.

"In other jurisdictions, it's not uncommon to see a very modern building pushed up against a heritage item, and there's a clear distinction between what's old and what's new, and we rather like that," he says.

"We get frustrated when we can't do it, or when we see someone who wants to do it and are thwarted in their attempt."

In the case of The Manuka Hotel That Wasn't, the plan was pinged by a cover-all sentence in the legislation that mandates that "any development has to be in keeping with the precinct and look the same" – in this case, a doubling of the building's size was clearly out before the proposal was even fully canvassed. It was, says Leeson, a good example of how the rules can discourage innovative responses to architectural conundrums.

But he concedes that there is also, quite rightly, anxiety surrounding building additions that don't date well.

"We all know that things come in and out of fashion, and what looks fantastic today, and modern and cutting edge, could look very old-fashioned in 10 or 20 years, and we're talking about very long timeframes for these precincts," he says.

We're talking in the firm's current Turner digs, a swish, white, trestle-tabled space the firm has occupied for the past three years since it outgrew its equally swell Manuka premises. With white spaces and walls covered in large-scale images of award-winning design projects, it feels every inch the architect's office, and Leeson himself, with his shock of grey hair, looks every bit the architect.

In fact, he says, he was always the type of kid who liked building things. Growing up in Griffith, he loved making objects – a papier-mache canoe stands out in his memory. When he finished school, he got work at an engineering firm, both drafting and in the workshop, before landing a job in an architect's firm.

"I realised after a while that there's no future in just being a draftsman, for an architect in a country town. So my wife chucked in a good job, and we moved here," he says.

By then, he was already 30, and his wife, Lee Erickson, worked as a radiographer – on very good pay – in the local hospital. It wasn't an easy move at the time, but Leeson was intent on studying architecture. He secured a spot as a mature-aged student, at what was then the Canberra College of Advanced Education (now the University of Canberra).

Upon graduating, he took a job with Peter Freeman, an architect with a long-term involvement with architectural heritage and conservation, and worked with him for 10 years before striking out alone in 1996. He began with one employee, in an office in Endeavour House in Manuka; today, he employs 10 staff, including himself and Erickson as his office manager.

Although they've never worked for the "big end" of town, so to speak, or done any large iconic buildings, Leeson is more proud of the smaller, off-beat projects the firm has worked on, like M16 Artspace in Griffith, and a swath of public housing projects all over the city. He has no patience for the hastily knocked-up apartment complexes that seem to be spreading like a virus across the suburbs, although he can well understand the dilemma faced by developers in an industry at the mercy of the vagaries of the economy.

"I can see why it happens, but it's no excuse to build crap because it's going to be there for a very long time, especially under a body corporate regime – there's so little opportunity to make them better," he says.

"When cracking plaster and stuff on the walls never gets fixed … that's the stuff that worries me and annoys me."

Ultimately, he still derives great satisfaction from working on private houses, especially those cases where he's able to prove that bigger doesn't always mean better. A homeowner may have become convinced, somewhere along the way, that the only way forward is to tear down a house and start again, but Leeson is adamant that it isn't always the case.

And these days the firm has a variety of projects on the go at any one time: from private residences to public art precincts such as the Megalo print studios, to large-scale heritage conservation such as Gorman House and Ainslie Arts Centre, small restoration projects, and the minute structures that pop up in unlikely places. These he refers to as "more like objects than buildings, in a way". His favourite is a small pavilion in the War Veterans Gardens in the Woden Cemetery.

"I love this little building – I got so much enjoyment going through the design process," he says.

His stamp, in fact, is scattered all around town. The rotunda in Glebe Park? He did that, along with the toilets and the small stage nearby. And at the moment, he's trying to come up with a way to conserve a tree – yes, an old, fallen tree – that happens to have a surveyor's marking from the very earliest days of the capital.

When he first started up his practice, he was told he may be lucky to last five years. That was almost 20 years ago.

"I think [Lee] has said it was worth it, but it was tough for her while I was studying, because I took it incredibly seriously," he says, laughing sheepishly.

"When I think back, I didn't have to do that, but architecture is one of those things. There's no end point … You can always make the design that little bit better no matter how far you take it. It's never perfect. At some point though, you've got to make the decision … There are these forced end points at every turn in architecture."

The Canberra and Region Heritage Festival runs from April 11-26.