The National Capital Authority (NCA) is investigating whether it should tighten up planning policies to stop higher density developments "eroding" the character of Canberra's most iconic areas.

The NCA has been responsible for a pocket of residential zone between National Circuit and State Circle for around a decade.

Their planning provisions governed the redevelopment of houses on large blocks of land, originally designed under the garden city philosophy, but in recent years there hs been community disquiet about the density of new developments.

"The NCA is aware of community concerns arising from the construction of a number of multi-unit residential development proposals in these areas," an NCA spokeswoman said.

"These, in part, stem from a concern that the landscape and streetscape character of the area is being eroded and appearance of new buildings is inconsistent with that of the rest of the suburb."

The NCA's chief planner Andrew Smith told a joint standing committee last week the blocks in the residential zone could be up to 3000 square metres, with up to six houses being built on what was one parcel of land.

"With that level of development, all the green space on the sites disappears and the mature treescape goes, and that is not the sort of outcome that we are seeking to achieve," he said.

"Certainly there can be some increase in density, but it was not intended to be a sea of townhouses and concrete driveways."

While the investigation was not far enough advanced for the NCA to say what changes there would be, Member for Canberra Gai Brodtmann wants the area afforded similar protections to the suburb of Reid.

Reid was built on garden planning principles of low-density housing that connects communities through gardens and reserves.

"It seems like every week I drive around there's another old home that's being pulled down in the inner south to make way for a new home for a dual occupancy and I'm becoming increasingly concerned about Canberra's loss of heritage and of our significant buildings," Ms Brodtmann said.

"We're a relatively young city but we're a city that has been designed on a very specific philosophy and I'm concerned about the impact these new homes and dual occupancy are having on the streetscape in the inner south and the concept of the garden city."

Talbot Street resident Megan Dick actually grew up in the house she now calls home. She's watched the surrounding streets change rapidly in the past six years.

"We've had two subdivisions, one on the corner block and one further up the street but more so around the corner in Somers Crescent, they've done a few subdivisions there, they've squeezed four houses onto one block and they're doing the same on the corner," she said.

"We're really unhappy about it, it seems to be changing the nature of the whole area. We loved the gardens and the trees and the feeling of space but when they stick four on a block it's just gone. It looks like it could be anywhere else."

However Andrew Chamberlain, a senior sales agent at Blackshaw Manuka, pointed out number of houses within NCA's jurisdiction was quite small and did not have a "tremendous amount of architectural significance" compared to neighbouring Blandfordia 4.

Blandfordia 4, which is now Forrest and Deakin, was home to three prime ministers and has an eclectic architectural style as well as its historical significance for Canberra.

Like Reid, Blandfordia was designed on the garden city principles and while it is also governed by heritage guidelines, it lies across the road from homes on NCA land which aren't afforded the same protection.

"Along National Circuit you have NCA on the northern side, you have Blandfordia on the southern side, there are rules that apply differently on one side of the road than the other," he said.

"Depending on your design criteria you can do dual occupancy on the NCA side but that is not permissible on the Blandfordia side of National Circuit."

While it would be ideal to preserve the garden city, a shortage of housing stock within sought-after areas made knock down and rebuilds an attractive proposition, Ron Bell, chief executive of the Real Estate Institute of the ACT, said.

"As Canberra grows, and we're heading very quickly towards 450,000 people, there's a lot of demand and it's increasing for older style homes on very large blocks that provide a lot of amenity for things like hospitals and schools and good shopping centres," Mr Bell said.

"If you witnessed what happened in Yarralumla a few weeks ago a lot of people were bidding for a house which eventually sold for $5.5 million."

Ms Brodtmann said she was sensitive to the need for more housing but felt there were some elements of old Canberra that needed to be preserved.

"I'm not wanting to set the place in aspic, I do understand the need to modernise and progress but we also need to be mindful that this is a very precious city and it's got very precious design philosophies behind it and we need to be mindful of it," she said.

The findings of the investigation will be delivered to the NCA board in early 2017.