The description of Barton Cottage as being 'defective'

As a house, Barton Cottage, though small, was comfortable and compact; but as a cottage it was defective, for the building was regular, the roof was tiled, the window shutters were not painted green, nor were the walls covered with honeysuckles. A narrow passage led directly through the house into the garden behind.

must come from Marianne, don't you think?

I've been wondering where she got such a notion, that it required an irregular type of building with green shutters and overgrown with honeysuckle. Many of Marianne's ideas about taste in drawing come from William Gilpin. It is clear that Jane Austen was familiar with his ideas because she even has Elizabeth Bennet make a joke about 'the picturesque will not admit a third' as a playful insult in P&P.

Gilpin liked unspoiled landscapes and could not approve of cottages that did not seem to blend in with the surroundings and, as he called it, "the kind of beauty that would look well in a picture".

The honeysuckle growing over the walls, the painted green shutters and the roof, possibly thatched as cottages often were, would blend in with the nature surrounding it.

Marianne's favourite, William Cowper, wrote a poem called 'The Cottage, Called the Peasant's Nest' in his work The Task, Book I, The Sofa.

In it he writes, in part,

A cottage, whither oft we since repair:
'Tis perched upon the green hill-top, but close
Environed with a ring of branching elms
That overhang the thatch, itself unseen
Peeps at the vale below; so thick beset
With foliage of such dark redundant growth,
I called the low-roofed lodge the peasant's nest.

Barton Cottage was not planned to appeal to that sort of sensibility--instead it seems to have been planned with more sense: a good location, well built, tiled roof that would withstand more than thatch might.

It's interesting the way the cottage might be seen as a concrete symbol of Sense and Sensibility