90 minutes to make connecting flight at Heathrow?

If I am reading this right you don't have a connecting flight as such but two separately booked flights with 90 minutes to transfer between them.

If you don't have checked bags & the flight arrives on time you will possibly make it on an airside transfer most of the time, with a lot of stress. You could easily though fail as both terminals are large and depending on which gates your flights are at you could have a considerable walk, as well as the inter terminal bus transfer. You will also need to pass security.

I think you have less chance to make it if you make a landside transfer.

I'm not sure what you mean by airside transfer. Can we walk through terminal 3 to terminal 2...is that what you mean? Would we still have to go through customs? We have carry-on bags so that is not an issue, but would we still have to go through security again? (Have only been at Heathrow once before and did not have to make any air connections.). Thanks for your response.

If this is two separate tickets, the chance of you making that connection is not that big.

Arriving at Heathrow requires going thru immigration control, then security as soon as you land, before you can get to the next flight, whether in a different terminal or even in the same terminal. Having to change terminals certainly adds to the potential dilemma. Having the flight land on-time at Heathrow (or even early?) could make all the difference in making the connecting flight. Be prepared to notify airport staff of your situation - we’ve gotten bumped up in the security line when we had a late flight and tight connection.

This is from the British Airways Website:

We recommend that you allow the following minimum connection times:
1 hour for connections within the same Heathrow terminal
1 hour 30 minutes for connections that require travel between terminals at Heathrow

So that airline suggests that 90 minutes will work, but don’t delay getting off the plane, thru immigration/passport control, then thru security, and on to the other terminal. Again, look for an airport official if you need help.

Worst case scenario, get yourself a new paddle (ticket), if you don’t make the Lufthansa connection and are up the creek at Heathrow. There’s a BA flight tomorrow evening (not that there’d definitely be one with available seats the day you land, but at least there appear to be options) directly to Hamburg, less than $150. Maybe a same-day flight would still get you to your final destination, and not cost an arm and both legs.

My experience with Heathrow is: you will not have enough time to change terminals. This is a huge airport with tons of people passing thru each day leading to very long security lines. If just changing gates in one terminal I would still worry about 90 minutes, but changing terminals is another kettel of fish. Try to change your flight times if possible, not a refund. Outside of that, request assistance if one of your party can qualify, then they will cut the line with the wheel chair and/or go a back route to make sure you catch your flight. Good luck J

Here is something to remember. When a flight is changed by that large of an amount, it is always refundable. You can cancel if the suggested flight is not acceptable and the airline must refund.

What people are asking you is if both flights are on one record locator? This means they were purchased as one purchase/ticket and not two different purchases/tickets. This matters quite a bit. If they are one ticket, you could take the gamble knowing that the airline would have to get you to Hamburg on next available flight. If they are two tickets, it is too risky.

90 minutes seems a little difficult to believe it will happen. It could, but it would be very stressful. I would use the
Flight Connections webpage for Heathrow and enter your flights to see what they say.

Go to:

http://www.heathrow.com/flight-connections

and plug in your flight information. It will give you step by step instructions.

When you arrive at T3, follow the purple flight connections signs. You do not have to go through immigration. You will be taken by free shuttle bus between terminals. You then go through security at T2, and then go to your gate. If you try to go through immigration and customs you will not make it. You are a transferring passenger so no need to do so.

Heathrow flight connections says it can be done in 75 minutes. That's assuming everything goes as plan.

No one can predict what airline or what flights will change.

The information provided by Frank II assumes you have your boarding passes for the Lufthansa flight in your hands. You will not be able to transfer airside to Terminal 2 and go through security there without your boarding passes for the next flight. But as long as you have those, you can use the Flight Connections path and avoid going through the passport control.

the OP has asked this question asked twice with some answers here and some there.

There is

https://community.ricksteves.com/travel-forum/transportation/schedule-change-and-tight-connection