Footnote: I hope this is the correct/ current regulation. Published date is 25 January 2015.
Your question is covered (for EASA regulations) in the document Certification Specifications and Guidance Material for Aerodromes Design (link) in two sections:
- Taxiway Slope (CS ADR-DSN.D.265)
- Taxiway Slope changes (CS ADR-DSN.D.270)
CS ADR-DSN.D.265 Longitudinal slopes on taxiways
(a) The safety objective of limiting the longitudinal taxiway slope
is to enable stabilised safe use of taxiway by an aircraft.
(b) The longitudinal slope of a taxiway should not exceed:
- 1.5% where the code letter is C, D, E, or F; and
- 3% where the code letter is A or B.
CS ADR-DSN.D.270 Longitudinal slope changes on taxiways
(a) The safety objective of limiting the longitudinal taxiway slope
changes is to avoid damage of aircraft and to enable safe use of
taxiway by an aircraft.
(b) Where slope changes on a taxiway cannot be avoided, the transition
from one slope to another slope should be accomplished by a curved
surface with a rate of change not exceeding:
- 1% per 30m (minimum radius of curvature of 3000m) where the code letter is C, D, E,or F; and
- 1% per 25m (minimum radius of curvature of 2500m) where the code letter is A or B.
(c) Where slope changes in (b)(1) and (2) are not achieved and
slopes on a taxiway cannot be avoided, the transition from one
slope to another slope should be accomplished by a curved
surface which should allow the safe operation of all aircraft in all
weather conditions.
The letters refer to the airport categorization.