Why couldn't Sirius leave Grimmauld Place in the Order of the Phoenix?

Question

After the events of Prisoner of Azkaban, Sirius spends the best part of a year (Goblet of Fire) living in a cave, and during the Order of the Phoenix lives in his family's house, Grimmauld Place.

It's pretty obvious that he hates Grimmauld Place, but Dumbledore made him stay there:

“Yeah, he did hate [the house]!” said Harry, his voice cracking, turning his back on Dumbledore and walking away. The sun was bright inside the room now, and the eyes of all the portraits followed him as he walked, without realizing what he was doing, without seeing the office at all. “You made him stay shut up in that house and he hated it, that’s why he wanted to get out last night—” — Order of the Phoenix, chapter 37

At the beginning of the book, he transforms into Padfoot to go and see Harry off at the station, but this is considered dangerous, as the Death Eaters know of his disguise from Pettigrew, and the Ministry is still using him as a scapegoat when things go wrong (e.g. the Death Eaters escape from Azkaban).

Granted, he is also less useful than his Order of the Phoenix peers, who had jobs at the Ministry and other ways of obtaining information, but it's still heavily implied that he'd have liked to go out for a stroll and help where possible.

Why couldn't he have left with a different disguise?

We see multiple options for disguises in the books:

• Invisibility Cloaks
• Polyjuice Potion
• Transfiguration (e.g. Ron's disguise as the invented Dragomir Despard during their Gringott's break-in)

Is there a reason given for why any of these, less "spottable" disguises, weren't used to allow Sirius to go out and about?

Answer 1

There are very few real invisibility cloaks. Most of them wear off after a while, while the few that did work were being put to good use (Harry needed his for his own protection, Moody needed his for work for the Order). Plus there was the fact that Sirius could not perform any real work for the Order, since most of the magic world still believed he was a mass murdering Voldemort supporter. So he would have needed an invisibility cloak all for himself just to take a stroll, which is a humongous waste of something as rare as an invisibility cloak.

As for Polyjuice Potion, there are similar reasons. The potion is not easy to make and its ingredients are not easy to acquire. It would be expending tremendous effort just to let Sirius take a few walks.

As for the Transfiguration, it did alter Ron's appearance to some extent. But at that time, they were also relying on Bellatrix's aura(as played by Hermione) to cover for Ron. Not to mention that nobody would expect Ron to just waltz down Diagon Alley when he was one of the most wanted wizards at that time. Sirius could have used Transfiguration, but anyone who knew Sirius might still be able to discern him. Since they were using Grimmauld Place as headquarters, they had to put everything into protecting it, and letting even a disguised Sirius out might have spoiled all the effort. Narcissa Malfoy(Narcissa Black before marriage), Sirius' cousin, could just send someone to check (or maybe go herself) and see if Sirius was hanging around there, especially since he had been recognized by Lucius Malfoy at the train station when he went to drop Harry off at the start of the term. Dumbledore wanted to take absolutely no chances, and so ordered Sirius to not leave the house.

Of course, Dumbledore later admits it was a mistake. Which means he realized he could have let Sirius out by some means or the other. He hadn't given much thought as to how Sirius could disguise himself because he hadn't foreseen the dangers of keeping Sirius locked up in his old house.

So yes, Sirius could have been allowed out by some method(maybe even the ones you mentioned), but no one thought it worth the trouble.

Answer 2

The answer may simply be "because Dumbledore said no"

Dumbledore commands respect among his subordinates and him simply telling Sirius sternly to not leave the house may be enough for him to obey

The point was to keep Sirius hidden because he was a target for both the Ministry (they still paint him as a criminal) and the DE (they possibly know through Snape that Dumbledore has revived the OOtP and Sirius is an active member)

Moreover Sirius being captured by Voldemort would have made Harry go out looking for him (as evidenced by the fact that this is indeed what he does when he thinks Sirius is captured) which is something Dumbledore definite doesn't want happening

All in all, it was because Dumbledore wouldn't allow it, not because of the limitations of any kind of magic trickery. As you pointed out there were tons of other things Sirius could have done to secretly leave the house.