In a case of new construction you typically have 3 options -
1.) Obtain a construction loan. In this case you are paying interest on the funds disbursed as the builder takes draws to build the home. This is typically the least desirable to the buyer (since you are paying interest while the home is being built, but you cannot live there). The positive is that it allows a long term rate lock, although that lock comes at a premium since the lender has to commit the loan funds well in advance of having a completed loan. It sounds like you are not pursuing this option from the message however, so there is no real need to discuss this option further.
2.) Float. Which you mentioned as one available option. You can simply wait to start your loan application, or start your application but not lock the rate, until you get closer to the construction being completed. The upside is that the closer you get to closing the lower the rate lock add on premiums will be, as the lender will not need to forecast a lock so far in the future. The downside is that obviously, should rates rise you could lose all of the gain or more, and be stuck with a higher rate than if you had taken a lock.
3.) Look for a long term lock. This will be similar in lock term to a construction loan however you will pay no interest and the loan will not start until the home is habitable, an appraisal is done, etc. At closing you are essentially purchasing the completed home from the builder with these funds. The plus is that you know what you are getting, the negative is that you pay a premium for locking in long term.
Looks like 3 is your best option, however there are a few things to be aware of - 1.) If you are only putting 10% down your loan will require approval from a PMI company. Even with excellent credit, PMI has become more difficult to obtain, and there is no telling where the guidelines might be by next February, when the PMI company will actually review the file. They will not typically make such long term forward commitments as a bank. 2.) Every long term lock option I have ever seen (unless it is builder financed) requires a 1% deposit at application. This is because it costs the bank to commit those funds 6-8 months out. If you close the loan, typically the 1% is credited to you at closing, but if you walk, you lose the 1%. That basically becomes the fee for tying up the banks money. A lender will normally call this a 'non refundable lock-in fee' or something to that effect. It is something you will typically come across anytime you lock for greater than 120 days. The last potential point is a float down option. Lenders also understand that when you are committing so far in advance that should rates get drastically better, you are going to want a better deal. To that end, most long term locks have a 1 time float down option. Sometimes there is an extra fee to do this, but it is probably your safest bet. It would work like this - let's say you lock at 5.5% now, simply to have the security of knowing your rate won't be any higher than that. Now lets say in December you can lock a 60 day loan at 4.875%, the bank will let you 'float down' to a lower rate for a fee. Each policy can vary slightly lender to lender, some may allow you to do it for a fee, others may middle the difference (instead of getting 4.875% you get 5% or 5.125%, still better than 5.5% but not as good as if you had rolled the dice). Regardless, make sure that you know all the ins and outs of a floatdown if you go with a long term lock option.
As for any alternative programs to what is outlined above, there really aren't any. What is listed above are the typical options available to you, none is as good as buying an already built home today, but that wouldn't require the long term lock so unfortunately your options become limited. Hope this helped,
Antonio